Novak Djokovic, the best player in the world, against Roger Federer, the best player ever.
A 18:00 GMT start, best of three sets, and one to blank out the diary for.
We'll see you there. Can't wait. Bye for now.
Post update
Federer 7-5 6-3 Wawrinka
The greatest risk to Roger Federer's place in tomorrow's final was probably an repetitive strain injury from the post-match autograph signing.
A fevered huddle, bobbing with giant ball and books, collects at the exit and the Swiss takes his time to sign plenty before admitting defeat as more and more join the queue.
Post update
Federer 7-5 6-3 Wawrinka
When he says that Novak Djokovic should be out already, Roger Federer is referring to his win over the Serb in the group stages earlier this week.
That win edged Federer 22-21 ahead in their head-to-head record.
I don't think it is going to be as straightforward as that 7-5 6-2 success on Sunday.
"Novak should be knocked out by now!"
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Roger Federer: "A great match, I really enjoyed it. It will be a perfect ending only if I win. Novak should be knocked out by now! No, I am joking. He played a great match against Rafael Nadal earlier today and I'm sure it will be a fantastic meeting."
Game, set and match
Federer 7-5 6-3 Wawrinka
ReutersCopyright: Reuters
Love-forty and three match points.
And, just like in the match as a whole, Roger Federer does not need to do that much to seal it.
A hoppy second serve should be returned in court, probably with interest.
Instead Stan frames one clean over the baseline to start the death march to the locker room.
Kind words at the net between the two.
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Wawrinka holds
*Federer 7-5 5-3 Wawrinka
ReutersCopyright: Reuters
Stan Wawrinka sees off a match point with an off forehand that sprays away for a winner.
Roger is not letting him rest just yet though. He keeps fetching and harrying to knock the world number four back from game point.
Finally Stan makes it over the line.
If he struggled that hard off his own serve though, what chance does he stand in the next game as Federer serves for the match?
Wawrinka taken to deuce
Federer 7-5 5-2 Wawrinka*
Love-thirty down and staring the exit door in the face, Stan Wawrinka's pride is stirred.
He unloads with three successive aces. Mighty serving. The crowd groan as his first serve is snagged in the tape on the next point.
No escape in style. Instead Stan lumps a backhand into the net to take us to deuce.
Federer 7-5 5-2 Wawrinka*
EPACopyright: EPA
Roger Federer has cruise control engaged and his feet up on the dashboard.
Another hold to love and Stan Wawrinka looks like a man who is here in body, but whose spirit and belief is already back in the locker room.
He serves to stay in the match next.
*Federer 7-5 4-2 Wawrinka
AFPCopyright: AFP
Stan Wawrinka keeps his end up, getting Roger Federer in a tangle on the baseline with a wrong-footing flat backhand on the game's final point.
He has to do more on return though if he is to get a chance to turn this match around.
Thierry Henry is in the house by the way and, after being picked out by the big screen a couple of times finally obliges with a half-hearted wave to the crowd.
Federer 7-5 4-1 Wawrinka*
ReutersCopyright: Reuters
Roger Federer comes under the service spotlight at 30-30.
That is the way to do it. Successive aces see off the danger.
It has to be said though, that neither of these men are at the level that Novak Djokovic set with a sparkling performance against Rafael Nadal earlier in the day.
*Federer 7-5 3-1 Wawrinka
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Stan Wawrinka is throwing the kitchen sink at the opposite end, but the balance has disappeared from his game.
He is charging in behind nothing-much groundstrokes and overcooking long as he goes for too much power.
Federer has 15-40 and two break points.
But Wawrinka yanks back on the joystick and pulls himself out of the tailspin just in time.
He puts away a couple of volleys in assured style to come out of that scrape.
Stars in their eyes
David Law
BBC commentator
We're getting a cheer for Hugh Grant. We're getting one for David Beckham, oh and Thierry Henry.
AFPCopyright: AFP
Federer holds
Federer 7-5 3-0 Wawrinka*
ReutersCopyright: Reuters
Roger Federer rides out that little bit of turbulence. He sees off a break point and coasts back to the sanctuary of his courtside stool with his serve intact.
It is only one break, but it looks like chasm on the scoreboard. He is half way there in this second set and, in the words of Bon Jovi, it is Stan Wawrinka living on a prayer.
Federer taken to deuce
*Federer 7-5 2-0 Wawrinka
EPACopyright: EPA
Roger Federer is dancing into the net at the merest invite and, even when the chance presents itself, Stan Wawrinka is struggling to squeeze the ball by his man and into court.
Hang on. That is more like it. A superb short cushioned angle from Stan and a Federer forehand into the net and we are at deuce.
Federer breaks
*Federer 7-5 2-0 Wawrinka
Roger Federer's wrists must have suspension like a monster truck.
He soaks up the venom from a vicious Stan Wawrinka forehand and drops a delicate volley out of reach.
Wawrinka's game is going pear-shaped suddenly. Errors cropping up like an infestation and he drills into the top part of the net to hand over the early break in the second set.
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Federer 7-5 1-0 Wawrinka*
Roger Federer's concentration wavers.
A double fault puts him into a spot of 0-30-style bother.
He looks as vulnerable as a peach in a blender in the next rally, but Wawrinka, with a clear line of sight down the line, sends his forehand south into the net.
Federer makes good his escape, via a helpful net cord, and is over the hill and safe before Stan can really turn the screw.
Game and first set
*Federer 7-5 Wawrinka
AFPCopyright: AFP
Stan Wawrinka suddenly gets as crumbly as a week-old lump of Wensleydale.
He double-faults, blots long and then sees a cross-court winner whizz past him for 15-40. Federer with two set points.
And Stan Wawrinka marches to his doom like a lemming, coming to the net once more without placing enough pressure on Roger Federer.
His volley is into the net and that is the first set down the gurgler.
Federer 6-5 Wawrinka*
AFPCopyright: AFP
This train looks like the non-stopper express to tie-breakville.
Roger Federer dusts off a service game to love.
Meanwhile a group of Swiss fans are looking decidedly smug as their nation's finest go head to head, oleing their way into some synchronised scarf waving.
No sighting, as yet, of any cowbells. Which is a pity.
*Federer 5-5 Wawrinka
AFPCopyright: AFP
Post-modern? Meta?
I don't know what it is, but a member of the crowd has created a giant cardboard cut-out of their own tweet declaring their love for Rodge and is waving it about proudly right here in the real, physical, offline world.
People have won the Turner Prize for less.
Anyway, Stan Wawrinka serving to stay in the first set and he makes no mistake, ploughing through to 15 with power and purpose.
Federer 5-4 Wawrinka*
Roger Federer is starting to dictate terms from the baseline. He catches Stan Wawrinka wrong-footed as he directs a backhand down the narrow angle of the court.
Into the net he tip-toes at 40-15 up. Coming in behind the cover of some good deep hitting, he puts away the volley to put himself ahead on the scoreboard.
Federer breaks
*Federer 4-4 Wawrinka
Roger Federer is back to level pegging as Stan Wawrinka's radar gets a little wonky off the deck.
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Two break points Federer
Federer 3-4 Wawrinka*
Stan Wawrinka plants a groundstroke wide and then duffs down a double fault and all of a sudden he has 15-40 and two break points to defend...
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John McEnerney: McEnroe & Connors in 82, Jimbo walked the length of the court & got right in JohnMc's face. No love lost there only hatred.
JohnMc on JohnMc, who'd have thought it?
If we get an I Nastase on Ilie Nastase, we'll start getting suspicious...
Federer 3-4 Wawrinka*
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Federer delivers a solid service game to keep in touch.
After that little flurry of break opportunities earlier in the set, it has all gone a little quiet at the returner's end.
David Beckham is doing a double session by the way. The former England captain - who always makes the most of his Wimbledon Royal Box pass - has hung about for the evening session after watching Novak Djokovic beat Rafael Nadal this afternoon.
Post update
Miles Maclagan
British tennis coach on Radio 5 live sports extra
AFPCopyright: AFP
That's a great hold after the break from Wawrinka. Nice and comfortable after dropping the first point.
*Federer 2-4 Wawrinka
Stan Wawrinka's serve has proved as slippery as an eel so far. Roger Federer is struggling to pin it down, chasing out wide and lunging back towards the tee as the barrage comes skidding his way.
A hold to 15 and the break backed up to open up some clear blue water on the scoreboard.
Wawrinka breaks
Federer 2-3 Wawrinka*
AFPCopyright: AFP
The net cord clearly is a Wawrinka fan.
First it flicks a Wawrinka forehand up into Federer's face at the net and he cannot scrape back.
Then it diverts a Federer shot wide between the tramlines.
First blood to Stan.
Federer taken to deuce
*Federer 2-2 Wawrinka
A little ripple of concern.
The sort you might hear if someone spilt their tea over the headmaster at a parents' evening.
Roger Federer has caused it. His double fault has given Stan Wawrinka 15-30.
He redeems the damage, but then yanks a forehand wide to offer up deuce.
*Federer 2-2 Wawrinka
Stan Wawrinka's forehand is full of socking power.
Another stroke out of the meaty part of the strings, stretches Federer out of shape just when the world number three sniffs a chance at 15-30.
A blurring ace seals the deal for Stan.
A hushed atmosphere at the O2 Arena. The crowd are into every point, savouring them like a selection box.
Federer holds
Federer 2-1 Wawrinka*
Rex FeaturesCopyright: Rex Features
Roger Federer gives Stan Wawrinka the slip, lulling his man into a couple of errors to wriggle off the hook.
Federer taken to deuce
*Federer 1-1 Wawrinka
After 11 points on serve, Stan Wawrinka finally brings the tee-off dictatorship to an end with a lovely, short backhand punch down the line for 40-15.
This rebellion has caught alight a bit. A forehand winner and Federer a smidge wide and out of nowhere we have deuce.
Swiss timing
Piers NewberyCopyright: Piers Newbery
*Federer 1-1 Wawrinka
EPACopyright: EPA
And Stan Wawrinka picks that gauntlet right up.
A hold to love of his own with his serve delivering arcing exocets out wide and down the middle.
Game on.
Federer 1-0 Wawrinka*
Roger Federer doesn't do snarling. Far too messy for the man himself. So he lets his tennis do the aggression.
A hold to love, with three out of the four points won at the net.
Slap! That is the sound of a gauntlet being laid down.
*Federer 0-0 Wawrinka
The players return to their courtside seating to sort out some bottles, wipe down some brows and stretch off some final muscles.
Federer to serve...
Rex FeaturesCopyright: Rex Features
Sprint finish
British athlete's courtside view
Dina Asher-SmithCopyright: Dina Asher-Smith
Get Involved
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In honour of last November's rather public misunderstanding between Mirka Federer and Stan Wawrinka, we are looking for your favourite on-court spats tonight.
David Nalbandian and foot-level advertising? Tim Henman and innocent ball girls? We've got video of them both in one convenient click.
This match is live on BBC Radio 5 live sports extra. You can get across their coverage with a click at the top of this page on either the play icon or the live coverage tab.
Player out on court
Federer v Wawrinka
The umpire attempts to steal the scene at the net introductions asking, like a boxing referee, for "a clean fight" and "no blows below the belt".
Federer, ever the diplomat, manages to raise a watery smile.
Post update
Federer v Wawrinka
Piers Newbery
BBC Sport at the O2 Arena in London
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The largest contingent of obviously partial supporters at this afternoon's semi-final were not Spanish or Serbian, but Swiss.
Such is their confidence in Federer, and more latterly Wawrinka, that it seems every sports fan in Switzerland block books the semi-finals and final at the O2 every year.
They might not have enjoyed what they saw this afternoon, with Djokovic looking imperious, but if he is to be stopped it will be a Swiss who does it.
Post update
Federer v Wawrinka
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If Stan is nervous on his way to the O2 playing surface, he is hiding it well.
The Swiss number two is chatting amiably with an ATP official before taking a small mascot with a giant tennis ball by the hand and striding through the dry ice.
New Swiss order
Federer v Wawrinka
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Stan Wawrinka has lost all 11 of the pair's meetings on hard court and 17 out of 20 overall.
But Wawrinka has won two Grand Slams - the 2014 Australian Open and 2015 French Open - since Federer's most recent and took his compatriot apart in straight sets in the last eight on his way to his Roland Garros triumph.
"I think now we are both nervous when we enter the court. Before, it was only me," Wawrinka said in September.
Post update
Federer v Wawrinka
Roger Federer: "I have to bring the power myself. He is very powerful off both forehand and backhand and when I can attack I have to take that chance as well."
Post update
Federer v Wawrinka
The 02 Arena is filling up fast. Pocket of red and white all arouund.
They could be supporters of either Swiss man, but in all likelihood they are Fed-heads. That is just the way the crowd support crumbles when you are playing the great man.
The voice of the World Tour Finals is currently growling through the background to each man's season.
Roger Federer is shooting the breeze in the bowels of the stadium.
One eye off court
Federer v Wawrinka
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In public at least, Federer and Wawrinka patched things up.
They were reportedly ushered into an O2 sideroom immediately after the match to sort out their on-going bickering and teamed up a week later to win the Davis Cup against France.
My good friend and wife have fallen out. I don't know how to fix things. Help!
Dear reader,
Good news. I have particular experience of this tricky dilemma...
Bye for now
Right, the litter-pickers and court-sweepers have descended on the O2 Arena court after Germany's Alexander Zverev picks up the ATP's Star of Tomorrow award - handed out to the youngest player in the top 100.
Previous winners include Jo-Wilfried Tsonga, Gael Monfils, Pat Rafter, Milos Raonic and Andy Roddick.
No sign of Stan Wawrinka or Roger Federer though. Those two will compete in the second semi-final at 20:00 GMT. See you back here, just before then.
Bye for now.
Match stats
Djokovic 6-3 6-3 Nadal
Nadal-Djokovic
Aces: 3-4
Double-faults: 0-1
First serve %: 70-61
First serve pts won %: 54-89
Second serve pts won %: 69-61
Break points won: 0/0 - 3/5
Total points won: 41-58
Post update
Djokovic 6-3 6-3 Nadal
Novak Djokovic will be runaway favourite regardless if it is Roger Federer or Stan Wawrinka in the final tomorrow.
The Serb dropped only ten points across his 12 service games - only three behind his first serve - and never faced a single break point.
Weaknesses?
Errr... one double-fault?
"A great performance"
Djokovic 6-3 6-3 Nadal
Novak Djokovic: "It was a great performance from the very beginning. I was trying to be aggressive and dictate the play, and I managed to pull out my best game when it was needed the most.
"We have not changed our games radically since we met the first time. We both play on the baseline, him with more spin, me trying to flatten it out a bit more. It is roughly the same as when we met for the first time.
On who he would like to play in the final: "I prefer playing against a Swiss player! They played a great match last year, and it is going to be nice to watch either way and hopefully I can perform as well as I did today."
Game, set and match
Djokovic 6-3 6-3 Nadal
And the scoreboard is kind to Nadal.
Total dominance from Novak Djokovic who cleans up with two pinging winners which would be remarkable if they were not so regular off his racquet.
Djokovic uppercuts thin air in celebration. Nadal trudges back behind the scene in deep contemplation.
Nadal taken to deuce
Djokovic 6-3 5-3 Nadal*
Oh fruity!
Novak Djokovic bunts a glorious lob over Nadal at the net to bring up 30-30 and wife Jelena to her feet in applause.
I think even Djokovic's nearest and dearest might secretly root for their man's opponent at times, just to liven up the dinner-table discussion.
Another milimetre-perfect lob from Djokovic and to deuce we go...
*Djokovic 6-3 5-3 Nadal
AFPCopyright: AFP
Was this where the match was finally won?
A loose forehand at the back of the court gives Nadal a glimmer, a half-sniff at 15-15.
A supreme stretching volley from Djokovic, sportingly applauded by Nadal, takes the next point. Nadal then levels up to 30-30.
But metronome hitting - perfect length, dead-eyed accuracy - hustle the Spaniard out of the next.
Nadal flies too close to the wire and his forehand is snagged south as his hopes die.
*Djokovic 6-3 4-3 Nadal
Rafael Nadal keeps his cards on the table. but I think Novak Djokovic has done the maths and cashed out already.
The Serbian world number one has dropped only eight points on his serve across the whole match.
Nadal needs to win at least four more in one of the next two Djokovic service games to turn the tide.
King Canute-like odds.
Post update
Piers Newbery
BBC Sport at the O2 Arena in London
AFPCopyright: AFP
Djokovic has been at his ruthless best this afternoon, peppering the lines like nobody else can, and a significant improvement on serve this year is also paying off.
He might not send down the huge cannon-like deliveries of coach Boris Becker, but the Serb has been mightily tough to break all season. Nadal has managed just eight points on the Serb's serve so far this afternoon, and he needs to find at least four in one game or it's 'adios'.
Djokovic 6-3 4-2 Nadal*
AFPCopyright: AFP
Rafael Nadal is striding around at the back of the court, looking for some sort of inspiration from somewhere.
Instead a ball boy offers him a towel.
The Djokovic forehand is thumping like a steam engine piston.
Sledgehammer power combined with laser-guided accuracy as he scoops up another game, this time for the loss of one point.
Djokovic breaks
*Djokovic 6-3 3-2 Nadal
Novak Djokovic sits on his courtside stool, staring into the middle distance with his shoulders heaving.
He gradually cranked the pace and angles beyond Nadal's limits to snap his serve on the second break point.
And now he looks like he is mentally tightening his hold on the match. Djokovic seems like a jolly presence off court.
On it, he is remorseless.
Djokovic with two break points
Djokovic 6-3 2-2 Nadal*
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Rafael Nadal on the ugly end of 15-30. His first serve misses. A flurry of twitches and tweaks as he prepares to launch the second. It's in. Into a rally.
Djokovic stretches wide once, twice, three time to recover apparently lost causes.
And Nadal nets a half-volley pick-up in the front court.
Two break points Djokovic at 15-40.
Djokovic 6-3 2-2 Nadal*
Novak Djokovic's serve looks as impregnable as Fort Knox.
He delivers another game to love with Nadal struggling to make him sweat, never mind slip up.
The spotlight swings back on Nadal's far-flakier tee-off.
*Djokovic 6-3 1-2 Nadal
Novak Djokovic crunches a double-handed backhand across court - as flat as a skimmed stone - to sow a seed of doubt and bring up 30-15 on the Nadal serve.
Djokovic looks like he is backing himself to ram-raid the Nadal serve with a string of unstoppable winners rather than pilfer it with cunning and tactics.
He goes hunting the whitewash, but fails to land the killer punches as he goes for broke.
Djokovic 6-3 1-1 Nadal*
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Novak Djokovic ticks off another service game to love.
Rafael Nadal puffed his cheeks. Getting a toehold into the Serb's tee-off has so far left him stumped.
Nadal holds
*Djokovic 6-3 0-1 Nadal
AFPCopyright: AFP
Rafael Nadal throws Novak Djokovic off the scent this time, holding onto his serve with the hounds of hell snapping at his heels.
He is the hunted rather than hunter in this match though. And, like the mountie, Djokovic always gets him man.
Nadal saves break point, deuce
Djokovic 6-3 0-0 Nadal*
Nurse, scalpel.
Novak Djokovic pulls on the gown and gloves and starts disemboweling another Rafael Nadal service game.
Love-30 before Nadal has collected his second-set senses.
A backhand long and a bolshy serve from Nadal haul him back to 30-30, but Djokovic bullies him out of the next rally for 30-40 and break point.
A peppery serve into Djokovic's body save Nadal this time but he is not out of the woods by a long way.
Deuce.
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Post update
Game and first set
Djokovic 6-3 Nadal*
AFPCopyright: AFP
Life, death, taxes and Novak Djokovic being better than the man on the other side of the net.
The Serb is one of life's certainties. They could re-calibrate the nearby Greenwich clock to his game it is so reliable.
First set bagged up and, even against a competitor like Rafael Nadal, this feels like a contest that is destined for camp Djoko.
*Djokovic 5-3 Nadal
AFPCopyright: AFP
Rafael Nadal holds to love.
All very well, but it feels like a footnote in a set of Novak Djokovic dominance.
The Serb is keeping his powder dry for the final haul over the line in the next service game.
Post update
Piers Newbery
BBC Sport at the O2 Arena in London
Djokovic has had his run-ins with the crowd here and elsewhere over the years when he's felt they are not giving him his due - it's harsh being the best and yet always playing the bad guy to Nadal and Federer especially.
There is more vocal support for the Spaniard here again today, although some of that might be down to ticket holders wanting a lengthy contest. Djokovic appears less keen on hanging around.
EPACopyright: EPA
Djokovic 5-2 Nadal*
The last game with the old set of tennis balls.
Might they have slowed down? Might this be Rafael Nadal's chance to pounce and break back?
If it was, it has gone in a flash. A hold to love whistles past the Spaniard's raised eyebrows.
*Djokovic 4-2 Nadal
ReutersCopyright: Reuters
Rafael Nadal keeps his head above water with a hold to 15.
Novak Djokovic clearly feels there is mileage in the drop-shot today, sending another delicate slip of a shot into the net on the final point of the game as he attempts to test Nadal's movement up and down the court.
Djokovic is two service holds away and there is little sign of Nadal mounting a rebellion just yet.
Djokovic 4-1 Nadal*
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David Beckham - a man who looks so good even his own kids are in a perpetual state of awe.
The former England captain will have appreciated the picture-perfect way that Novak Djokovic polished off that game.
The Serb lured Rafael Nadal into the net with a dropshot with dampened strings and then sends a weighted lob cascading over his opponent and in.
Djokovic still up somewhere in the stratosphere.
*Djokovic 3-1 Nadal
Gymnast Louis Smith, TV reality star girlfriend Lucy Mecklenburgh and Olympic skeleton champion Lizzie Yarnold are in the celeb section.
They have just been brought bubbly in sponsored silver goblets. The only time I have been served a drink in a goblet was at a jousting show in Florida called Medieval Times. It was mead and it was great.
Rafael Nadal has steadied the ship with a service hold after coming under a spot of pressure at 15-30.
He needed that.
Djokovic 3-0 Nadal*
Rex FeaturesCopyright: Rex Features
No-one on this planet, never mind in this field, can live with Djokovic in this form.
The Serb has been immaculate so far. Nadal manages to prolong the game to 40-30 as he snips the outside of the sideline with a return.
But Djokovic is hitting cleaner, deeper and into narrow margins with relentless accuracy.
Djokovic breaks
*Djokovic 2-0 Nadal
A statement of intent.
Novak Djokovic chows down on two second serves - blasting away the first for a return winner before rolling a dead-eyed backhand down the line for 0-30.
Alarm bells clanging for Rafa.
Djokovic pings another sickly-sweet forehand down the line for 0-40 and three break points.
And he takes the first with an jaw-dropper of a backhand across court that is too good for Nadal's fabled forehand.
Getty ImagesCopyright: Getty Images
Djokovic 1-0 Nadal*
ReutersCopyright: Reuters
Novak Djokovic's serve is as sharp as a start-of-term stationary set.
An rifled ace opens the match, before a foxy slower-ball second serve catches Nadal out and brings up 30-15.
The first matador flourish from Rafael Nadal as he swats a lovely running forehand winner from out wide for 30-30.
Djokovic is diamond-encrusted though when it comes to the attritional rallies. The Serb wears down Nadal and finally picks out a thumping forehand down the line to close out the game.
Plenty of intrigue, cut and thrust to get your teeth into in those opening exchanges though...
Djokovic to serve
*Djokovic 0-0 Nadal
Getty ImagesCopyright: Getty Images
Both players pumping down the practice serves.
Neither has banished their entourages to the gods like Andy Murray did last night - Rafael Nadal's other half is down courtside alongside his sister.
Novak Djokovic will get first use of the service fuzz.
Nadal could return to top of game
Andrew Castle
BBC Sport
Rex FeaturesCopyright: Rex Features
"This tournament is the first time in a long time that it feels like Rafael Nadal could climb back to the very top of the game."
Rafa back to where he should be
Tim Henman
Former British number one on BBC One
"In the last five or six weeks Rafa has got his game back to where it should be.
"He has been more aggressive and flattened his backhand out. He has nothing to lose and lots to gain today."
Post update
Djokovic v Nadal
Knock-up underway
Djokovic v Nadal
AFPCopyright: AFP
The stadium emcee raps his vocal cords around Rafa Nadal's vowels with relish as he announces the players' arrival on court.
The players unwrap their bats in near darkness as the house lights are kept off and the dance music keeps pumping.
It is like a sweaty spinning class at the O2.
To the net. Rafa bouncing, Novak stretching hamstrings. Umpire Mohamed Lahyani issues final instructions and competition winner 'Sharon' flips the coin and poses for photos.
Knock-up under way.
BBC coverage
Djokovic v Nadal
BBC One
Quick, everyone over to BBC One.
A lovely sepia start to the television coverage, full of clenched fists and grimaces from past meetings between Novak Djokovic and Rafael Nadal.
Down to business and Andrew Castle has ticked the "era-defining rivalry" soundbite off your bingo card.
Get involved via the play icon at the top of this page.
Nadal the one to watch in 2016
Piers Newbery
BBC Sport at the O2 Arena in London
Rex FeaturesCopyright: Rex Features
"He showed what's to come in 2016, in my opinion.With some time to practise and reassess his game, he's going to be tough to beat in Australia. I believe that." Nadal's resurgence has not gone unnoticed by the rest of the field and Roger Federer is predicting big things from his great rival next year. Nadal himself has been in an unusually cheerful mood all week in London and is planning an intensive off-season, even by his standards. "I don't deserve a vacation," he joked. We think.
Swiss role
GettyCopyright: Getty
This is the day's first semi-final of course.
The evening session, appropriately enough for an arena set in the heart of London's financial engineroom, is an all Swiss affair.
Tennis' Dorian Gray Roger Federer takes on the majestic backhand that comes attached to Stan Wawrinka.
The first serve crashes down in that one shortly after 20:00 GMT.
Skill and crossed racquets
Piers Newbery
BBC Sport at the O2 Arena in London
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It's an icily cold day on the North Greenwich Peninsula, a place where the corpses of pirates were once hung in cages to warn off others. Now they play tennis in a giant tent, which is a much better idea. Semi-finals day has provided real drama here over the years, from Murray and Nadal's epic in 2010 to Federer and Wawrinka's falling out last year. There are 17,000 spectators now making their way through security fully expecting another thriller this afternoon.
Head to head
Djokovic v Nadal
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Since then Rafa has cut his hair, Novak has cut out gluten and one of the great rivalries in modern tennis has blossomed.
There have been 45 matches between the pair, with Nadal narrowly ahead 23 to 22 on the countback.
What awaits us this afternoon as they go head to head for a place in the World Tour Finals?
Back in the day
Djokovic v Nadal
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The first Tour meeting between Novak Djokovic and Rafael Nadal was back in June 2006 when Tim Henman and Greg Rusedski were still knocking around in the first week of the French Open.
Nadal and Djokovic met in the quarter-finals at Roland Garros with the Spaniard prevailing via a retirement when 6-4 6-4 up.
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Latest PostPost update
Federer 7-5 6-3 Wawrinka
So the season comes down to this.
Novak Djokovic, the best player in the world, against Roger Federer, the best player ever.
A 18:00 GMT start, best of three sets, and one to blank out the diary for.
We'll see you there. Can't wait. Bye for now.
Post update
Federer 7-5 6-3 Wawrinka
The greatest risk to Roger Federer's place in tomorrow's final was probably an repetitive strain injury from the post-match autograph signing.
A fevered huddle, bobbing with giant ball and books, collects at the exit and the Swiss takes his time to sign plenty before admitting defeat as more and more join the queue.
Post update
Federer 7-5 6-3 Wawrinka
When he says that Novak Djokovic should be out already, Roger Federer is referring to his win over the Serb in the group stages earlier this week.
That win edged Federer 22-21 ahead in their head-to-head record.
I don't think it is going to be as straightforward as that 7-5 6-2 success on Sunday.
"Novak should be knocked out by now!"
Roger Federer: "A great match, I really enjoyed it. It will be a perfect ending only if I win. Novak should be knocked out by now! No, I am joking. He played a great match against Rafael Nadal earlier today and I'm sure it will be a fantastic meeting."
Game, set and match
Federer 7-5 6-3 Wawrinka
Love-forty and three match points.
And, just like in the match as a whole, Roger Federer does not need to do that much to seal it.
A hoppy second serve should be returned in court, probably with interest.
Instead Stan frames one clean over the baseline to start the death march to the locker room.
Kind words at the net between the two.
Wawrinka holds
*Federer 7-5 5-3 Wawrinka
Stan Wawrinka sees off a match point with an off forehand that sprays away for a winner.
Roger is not letting him rest just yet though. He keeps fetching and harrying to knock the world number four back from game point.
Finally Stan makes it over the line.
If he struggled that hard off his own serve though, what chance does he stand in the next game as Federer serves for the match?
Wawrinka taken to deuce
Federer 7-5 5-2 Wawrinka*
Love-thirty down and staring the exit door in the face, Stan Wawrinka's pride is stirred.
He unloads with three successive aces. Mighty serving. The crowd groan as his first serve is snagged in the tape on the next point.
No escape in style. Instead Stan lumps a backhand into the net to take us to deuce.
Federer 7-5 5-2 Wawrinka*
Roger Federer has cruise control engaged and his feet up on the dashboard.
Another hold to love and Stan Wawrinka looks like a man who is here in body, but whose spirit and belief is already back in the locker room.
He serves to stay in the match next.
*Federer 7-5 4-2 Wawrinka
Stan Wawrinka keeps his end up, getting Roger Federer in a tangle on the baseline with a wrong-footing flat backhand on the game's final point.
He has to do more on return though if he is to get a chance to turn this match around.
Thierry Henry is in the house by the way and, after being picked out by the big screen a couple of times finally obliges with a half-hearted wave to the crowd.
Federer 7-5 4-1 Wawrinka*
Roger Federer comes under the service spotlight at 30-30.
That is the way to do it. Successive aces see off the danger.
It has to be said though, that neither of these men are at the level that Novak Djokovic set with a sparkling performance against Rafael Nadal earlier in the day.
*Federer 7-5 3-1 Wawrinka
Stan Wawrinka is throwing the kitchen sink at the opposite end, but the balance has disappeared from his game.
He is charging in behind nothing-much groundstrokes and overcooking long as he goes for too much power.
Federer has 15-40 and two break points.
But Wawrinka yanks back on the joystick and pulls himself out of the tailspin just in time.
He puts away a couple of volleys in assured style to come out of that scrape.
Stars in their eyes
David Law
BBC commentator
We're getting a cheer for Hugh Grant. We're getting one for David Beckham, oh and Thierry Henry.
Federer holds
Federer 7-5 3-0 Wawrinka*
Roger Federer rides out that little bit of turbulence. He sees off a break point and coasts back to the sanctuary of his courtside stool with his serve intact.
It is only one break, but it looks like chasm on the scoreboard. He is half way there in this second set and, in the words of Bon Jovi, it is Stan Wawrinka living on a prayer.
Federer taken to deuce
*Federer 7-5 2-0 Wawrinka
Roger Federer is dancing into the net at the merest invite and, even when the chance presents itself, Stan Wawrinka is struggling to squeeze the ball by his man and into court.
Hang on. That is more like it. A superb short cushioned angle from Stan and a Federer forehand into the net and we are at deuce.
Federer breaks
*Federer 7-5 2-0 Wawrinka
Roger Federer's wrists must have suspension like a monster truck.
He soaks up the venom from a vicious Stan Wawrinka forehand and drops a delicate volley out of reach.
Wawrinka's game is going pear-shaped suddenly. Errors cropping up like an infestation and he drills into the top part of the net to hand over the early break in the second set.
Federer 7-5 1-0 Wawrinka*
Roger Federer's concentration wavers.
A double fault puts him into a spot of 0-30-style bother.
He looks as vulnerable as a peach in a blender in the next rally, but Wawrinka, with a clear line of sight down the line, sends his forehand south into the net.
Federer makes good his escape, via a helpful net cord, and is over the hill and safe before Stan can really turn the screw.
Game and first set
*Federer 7-5 Wawrinka
Stan Wawrinka suddenly gets as crumbly as a week-old lump of Wensleydale.
He double-faults, blots long and then sees a cross-court winner whizz past him for 15-40. Federer with two set points.
And Stan Wawrinka marches to his doom like a lemming, coming to the net once more without placing enough pressure on Roger Federer.
His volley is into the net and that is the first set down the gurgler.
Federer 6-5 Wawrinka*
This train looks like the non-stopper express to tie-breakville.
Roger Federer dusts off a service game to love.
Meanwhile a group of Swiss fans are looking decidedly smug as their nation's finest go head to head, oleing their way into some synchronised scarf waving.
No sighting, as yet, of any cowbells. Which is a pity.
*Federer 5-5 Wawrinka
Post-modern? Meta?
I don't know what it is, but a member of the crowd has created a giant cardboard cut-out of their own tweet declaring their love for Rodge and is waving it about proudly right here in the real, physical, offline world.
People have won the Turner Prize for less.
Anyway, Stan Wawrinka serving to stay in the first set and he makes no mistake, ploughing through to 15 with power and purpose.
Federer 5-4 Wawrinka*
Roger Federer is starting to dictate terms from the baseline. He catches Stan Wawrinka wrong-footed as he directs a backhand down the narrow angle of the court.
Into the net he tip-toes at 40-15 up. Coming in behind the cover of some good deep hitting, he puts away the volley to put himself ahead on the scoreboard.
Federer breaks
*Federer 4-4 Wawrinka
Roger Federer is back to level pegging as Stan Wawrinka's radar gets a little wonky off the deck.
Two break points Federer
Federer 3-4 Wawrinka*
Stan Wawrinka plants a groundstroke wide and then duffs down a double fault and all of a sudden he has 15-40 and two break points to defend...
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John McEnerney: McEnroe & Connors in 82, Jimbo walked the length of the court & got right in JohnMc's face. No love lost there only hatred.
JohnMc on JohnMc, who'd have thought it?
If we get an I Nastase on Ilie Nastase, we'll start getting suspicious...
Federer 3-4 Wawrinka*
Federer delivers a solid service game to keep in touch.
After that little flurry of break opportunities earlier in the set, it has all gone a little quiet at the returner's end.
David Beckham is doing a double session by the way. The former England captain - who always makes the most of his Wimbledon Royal Box pass - has hung about for the evening session after watching Novak Djokovic beat Rafael Nadal this afternoon.
Post update
Miles Maclagan
British tennis coach on Radio 5 live sports extra
That's a great hold after the break from Wawrinka. Nice and comfortable after dropping the first point.
*Federer 2-4 Wawrinka
Stan Wawrinka's serve has proved as slippery as an eel so far. Roger Federer is struggling to pin it down, chasing out wide and lunging back towards the tee as the barrage comes skidding his way.
A hold to 15 and the break backed up to open up some clear blue water on the scoreboard.
Wawrinka breaks
Federer 2-3 Wawrinka*
The net cord clearly is a Wawrinka fan.
First it flicks a Wawrinka forehand up into Federer's face at the net and he cannot scrape back.
Then it diverts a Federer shot wide between the tramlines.
First blood to Stan.
Federer taken to deuce
*Federer 2-2 Wawrinka
A little ripple of concern.
The sort you might hear if someone spilt their tea over the headmaster at a parents' evening.
Roger Federer has caused it. His double fault has given Stan Wawrinka 15-30.
He redeems the damage, but then yanks a forehand wide to offer up deuce.
*Federer 2-2 Wawrinka
Stan Wawrinka's forehand is full of socking power.
Another stroke out of the meaty part of the strings, stretches Federer out of shape just when the world number three sniffs a chance at 15-30.
A blurring ace seals the deal for Stan.
A hushed atmosphere at the O2 Arena. The crowd are into every point, savouring them like a selection box.
Federer holds
Federer 2-1 Wawrinka*
Roger Federer gives Stan Wawrinka the slip, lulling his man into a couple of errors to wriggle off the hook.
Federer taken to deuce
*Federer 1-1 Wawrinka
After 11 points on serve, Stan Wawrinka finally brings the tee-off dictatorship to an end with a lovely, short backhand punch down the line for 40-15.
This rebellion has caught alight a bit. A forehand winner and Federer a smidge wide and out of nowhere we have deuce.
Swiss timing
*Federer 1-1 Wawrinka
And Stan Wawrinka picks that gauntlet right up.
A hold to love of his own with his serve delivering arcing exocets out wide and down the middle.
Game on.
Federer 1-0 Wawrinka*
Roger Federer doesn't do snarling. Far too messy for the man himself. So he lets his tennis do the aggression.
A hold to love, with three out of the four points won at the net.
Slap! That is the sound of a gauntlet being laid down.
*Federer 0-0 Wawrinka
The players return to their courtside seating to sort out some bottles, wipe down some brows and stretch off some final muscles.
Federer to serve...
Sprint finish
British athlete's courtside view
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In honour of last November's rather public misunderstanding between Mirka Federer and Stan Wawrinka, we are looking for your favourite on-court spats tonight.
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BBC coverage
Federer v Wawrinka
BBC Radio 5 Live Sports Extra
This match is live on BBC Radio 5 live sports extra. You can get across their coverage with a click at the top of this page on either the play icon or the live coverage tab.
Player out on court
Federer v Wawrinka
The umpire attempts to steal the scene at the net introductions asking, like a boxing referee, for "a clean fight" and "no blows below the belt".
Federer, ever the diplomat, manages to raise a watery smile.
Post update
Federer v Wawrinka
Piers Newbery
BBC Sport at the O2 Arena in London
The largest contingent of obviously partial supporters at this afternoon's semi-final were not Spanish or Serbian, but Swiss.
Such is their confidence in Federer, and more latterly Wawrinka, that it seems every sports fan in Switzerland block books the semi-finals and final at the O2 every year.
They might not have enjoyed what they saw this afternoon, with Djokovic looking imperious, but if he is to be stopped it will be a Swiss who does it.
Post update
Federer v Wawrinka
If Stan is nervous on his way to the O2 playing surface, he is hiding it well.
The Swiss number two is chatting amiably with an ATP official before taking a small mascot with a giant tennis ball by the hand and striding through the dry ice.
New Swiss order
Federer v Wawrinka
Stan Wawrinka has lost all 11 of the pair's meetings on hard court and 17 out of 20 overall.
But Wawrinka has won two Grand Slams - the 2014 Australian Open and 2015 French Open - since Federer's most recent and took his compatriot apart in straight sets in the last eight on his way to his Roland Garros triumph.
"I think now we are both nervous when we enter the court. Before, it was only me," Wawrinka said in September.
Post update
Federer v Wawrinka
Roger Federer: "I have to bring the power myself. He is very powerful off both forehand and backhand and when I can attack I have to take that chance as well."
Post update
Federer v Wawrinka
The 02 Arena is filling up fast. Pocket of red and white all arouund.
They could be supporters of either Swiss man, but in all likelihood they are Fed-heads. That is just the way the crowd support crumbles when you are playing the great man.
The voice of the World Tour Finals is currently growling through the background to each man's season.
Roger Federer is shooting the breeze in the bowels of the stadium.
One eye off court
Federer v Wawrinka
In public at least, Federer and Wawrinka patched things up.
They were reportedly ushered into an O2 sideroom immediately after the match to sort out their on-going bickering and teamed up a week later to win the Davis Cup against France.
But if it gets close.
If there is a controversial call.
If Wawrinka's girlfriend Croatian pro Donna Vekic - who is, in her own words, "strong-willed, determined and extremely competitive" - hears something she doesn't like.
Expect the snappers' lenses to swing to the players' box sharpish.
Mirka the merciless
Federer v Wawrinka
That voice belonged to Roger's wife Mirka.
After the match nobody was talking about Federer's victory.
Federer took the Arsene Wenger defence.
“It just flared up big time. I saw Stan and I saw Mirka, and I was in the middle of it,” he said.
“I didn’t even see what happened."
Twelve months ago
Federer v Wawrinka
Let's rewind things 12 months.
Another O2 Arena semi-final, but the same protagonists - Roger Federer playing Davis Cup team-mate and long-time mate Stan Wawrinka.
It's the 11th game of the third and deciding set. And it is tight.
Wawrinka, who has squandered three match points, turns away from the baseline as he is about to receive serve and complains to umpire Cedric Mourier.
“Not just before the serve, not just before the serve,” he says pointing to the courtside seats.
A voice comes back from the front row: "Crybaby"
Dear Roger...
Federer v Wawrinka
Dear Roger,
My good friend and wife have fallen out. I don't know how to fix things. Help!
Dear reader,
Good news. I have particular experience of this tricky dilemma...
Bye for now
Right, the litter-pickers and court-sweepers have descended on the O2 Arena court after Germany's Alexander Zverev picks up the ATP's Star of Tomorrow award - handed out to the youngest player in the top 100.
Previous winners include Jo-Wilfried Tsonga, Gael Monfils, Pat Rafter, Milos Raonic and Andy Roddick.
No sign of Stan Wawrinka or Roger Federer though. Those two will compete in the second semi-final at 20:00 GMT. See you back here, just before then.
Bye for now.
Match stats
Djokovic 6-3 6-3 Nadal
Nadal-Djokovic
Aces: 3-4
Double-faults: 0-1
First serve %: 70-61
First serve pts won %: 54-89
Second serve pts won %: 69-61
Break points won: 0/0 - 3/5
Total points won: 41-58
Post update
Djokovic 6-3 6-3 Nadal
Novak Djokovic will be runaway favourite regardless if it is Roger Federer or Stan Wawrinka in the final tomorrow.
The Serb dropped only ten points across his 12 service games - only three behind his first serve - and never faced a single break point.
Weaknesses?
Errr... one double-fault?
"A great performance"
Djokovic 6-3 6-3 Nadal
Novak Djokovic: "It was a great performance from the very beginning. I was trying to be aggressive and dictate the play, and I managed to pull out my best game when it was needed the most.
"We have not changed our games radically since we met the first time. We both play on the baseline, him with more spin, me trying to flatten it out a bit more. It is roughly the same as when we met for the first time.
On who he would like to play in the final: "I prefer playing against a Swiss player! They played a great match last year, and it is going to be nice to watch either way and hopefully I can perform as well as I did today."
Game, set and match
Djokovic 6-3 6-3 Nadal
And the scoreboard is kind to Nadal.
Total dominance from Novak Djokovic who cleans up with two pinging winners which would be remarkable if they were not so regular off his racquet.
Djokovic uppercuts thin air in celebration. Nadal trudges back behind the scene in deep contemplation.
Nadal taken to deuce
Djokovic 6-3 5-3 Nadal*
Oh fruity!
Novak Djokovic bunts a glorious lob over Nadal at the net to bring up 30-30 and wife Jelena to her feet in applause.
I think even Djokovic's nearest and dearest might secretly root for their man's opponent at times, just to liven up the dinner-table discussion.
Another milimetre-perfect lob from Djokovic and to deuce we go...
*Djokovic 6-3 5-3 Nadal
Was this where the match was finally won?
A loose forehand at the back of the court gives Nadal a glimmer, a half-sniff at 15-15.
A supreme stretching volley from Djokovic, sportingly applauded by Nadal, takes the next point. Nadal then levels up to 30-30.
But metronome hitting - perfect length, dead-eyed accuracy - hustle the Spaniard out of the next.
Nadal flies too close to the wire and his forehand is snagged south as his hopes die.
*Djokovic 6-3 4-3 Nadal
Rafael Nadal keeps his cards on the table. but I think Novak Djokovic has done the maths and cashed out already.
The Serbian world number one has dropped only eight points on his serve across the whole match.
Nadal needs to win at least four more in one of the next two Djokovic service games to turn the tide.
King Canute-like odds.
Post update
Piers Newbery
BBC Sport at the O2 Arena in London
Djokovic has been at his ruthless best this afternoon, peppering the lines like nobody else can, and a significant improvement on serve this year is also paying off.
He might not send down the huge cannon-like deliveries of coach Boris Becker, but the Serb has been mightily tough to break all season. Nadal has managed just eight points on the Serb's serve so far this afternoon, and he needs to find at least four in one game or it's 'adios'.
Djokovic 6-3 4-2 Nadal*
Rafael Nadal is striding around at the back of the court, looking for some sort of inspiration from somewhere.
Instead a ball boy offers him a towel.
The Djokovic forehand is thumping like a steam engine piston.
Sledgehammer power combined with laser-guided accuracy as he scoops up another game, this time for the loss of one point.
Djokovic breaks
*Djokovic 6-3 3-2 Nadal
Novak Djokovic sits on his courtside stool, staring into the middle distance with his shoulders heaving.
He gradually cranked the pace and angles beyond Nadal's limits to snap his serve on the second break point.
And now he looks like he is mentally tightening his hold on the match. Djokovic seems like a jolly presence off court.
On it, he is remorseless.
Djokovic with two break points
Djokovic 6-3 2-2 Nadal*
Rafael Nadal on the ugly end of 15-30. His first serve misses. A flurry of twitches and tweaks as he prepares to launch the second. It's in. Into a rally.
Djokovic stretches wide once, twice, three time to recover apparently lost causes.
And Nadal nets a half-volley pick-up in the front court.
Two break points Djokovic at 15-40.
Djokovic 6-3 2-2 Nadal*
Novak Djokovic's serve looks as impregnable as Fort Knox.
He delivers another game to love with Nadal struggling to make him sweat, never mind slip up.
The spotlight swings back on Nadal's far-flakier tee-off.
*Djokovic 6-3 1-2 Nadal
Novak Djokovic crunches a double-handed backhand across court - as flat as a skimmed stone - to sow a seed of doubt and bring up 30-15 on the Nadal serve.
Djokovic looks like he is backing himself to ram-raid the Nadal serve with a string of unstoppable winners rather than pilfer it with cunning and tactics.
He goes hunting the whitewash, but fails to land the killer punches as he goes for broke.
Djokovic 6-3 1-1 Nadal*
Novak Djokovic ticks off another service game to love.
Rafael Nadal puffed his cheeks. Getting a toehold into the Serb's tee-off has so far left him stumped.
Nadal holds
*Djokovic 6-3 0-1 Nadal
Rafael Nadal throws Novak Djokovic off the scent this time, holding onto his serve with the hounds of hell snapping at his heels.
He is the hunted rather than hunter in this match though. And, like the mountie, Djokovic always gets him man.
Nadal saves break point, deuce
Djokovic 6-3 0-0 Nadal*
Nurse, scalpel.
Novak Djokovic pulls on the gown and gloves and starts disemboweling another Rafael Nadal service game.
Love-30 before Nadal has collected his second-set senses.
A backhand long and a bolshy serve from Nadal haul him back to 30-30, but Djokovic bullies him out of the next rally for 30-40 and break point.
A peppery serve into Djokovic's body save Nadal this time but he is not out of the woods by a long way.
Deuce.
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Post update
Game and first set
Djokovic 6-3 Nadal*
Life, death, taxes and Novak Djokovic being better than the man on the other side of the net.
The Serb is one of life's certainties. They could re-calibrate the nearby Greenwich clock to his game it is so reliable.
First set bagged up and, even against a competitor like Rafael Nadal, this feels like a contest that is destined for camp Djoko.
*Djokovic 5-3 Nadal
Rafael Nadal holds to love.
All very well, but it feels like a footnote in a set of Novak Djokovic dominance.
The Serb is keeping his powder dry for the final haul over the line in the next service game.
Post update
Piers Newbery
BBC Sport at the O2 Arena in London
Djokovic has had his run-ins with the crowd here and elsewhere over the years when he's felt they are not giving him his due - it's harsh being the best and yet always playing the bad guy to Nadal and Federer especially.
There is more vocal support for the Spaniard here again today, although some of that might be down to ticket holders wanting a lengthy contest. Djokovic appears less keen on hanging around.
Djokovic 5-2 Nadal*
The last game with the old set of tennis balls.
Might they have slowed down? Might this be Rafael Nadal's chance to pounce and break back?
If it was, it has gone in a flash. A hold to love whistles past the Spaniard's raised eyebrows.
*Djokovic 4-2 Nadal
Rafael Nadal keeps his head above water with a hold to 15.
Novak Djokovic clearly feels there is mileage in the drop-shot today, sending another delicate slip of a shot into the net on the final point of the game as he attempts to test Nadal's movement up and down the court.
Djokovic is two service holds away and there is little sign of Nadal mounting a rebellion just yet.
Djokovic 4-1 Nadal*
David Beckham - a man who looks so good even his own kids are in a perpetual state of awe.
The former England captain will have appreciated the picture-perfect way that Novak Djokovic polished off that game.
The Serb lured Rafael Nadal into the net with a dropshot with dampened strings and then sends a weighted lob cascading over his opponent and in.
Djokovic still up somewhere in the stratosphere.
*Djokovic 3-1 Nadal
Gymnast Louis Smith, TV reality star girlfriend Lucy Mecklenburgh and Olympic skeleton champion Lizzie Yarnold are in the celeb section.
They have just been brought bubbly in sponsored silver goblets. The only time I have been served a drink in a goblet was at a jousting show in Florida called Medieval Times. It was mead and it was great.
Rafael Nadal has steadied the ship with a service hold after coming under a spot of pressure at 15-30.
He needed that.
Djokovic 3-0 Nadal*
No-one on this planet, never mind in this field, can live with Djokovic in this form.
The Serb has been immaculate so far. Nadal manages to prolong the game to 40-30 as he snips the outside of the sideline with a return.
But Djokovic is hitting cleaner, deeper and into narrow margins with relentless accuracy.
Djokovic breaks
*Djokovic 2-0 Nadal
A statement of intent.
Novak Djokovic chows down on two second serves - blasting away the first for a return winner before rolling a dead-eyed backhand down the line for 0-30.
Alarm bells clanging for Rafa.
Djokovic pings another sickly-sweet forehand down the line for 0-40 and three break points.
And he takes the first with an jaw-dropper of a backhand across court that is too good for Nadal's fabled forehand.
Djokovic 1-0 Nadal*
Novak Djokovic's serve is as sharp as a start-of-term stationary set.
An rifled ace opens the match, before a foxy slower-ball second serve catches Nadal out and brings up 30-15.
The first matador flourish from Rafael Nadal as he swats a lovely running forehand winner from out wide for 30-30.
Djokovic is diamond-encrusted though when it comes to the attritional rallies. The Serb wears down Nadal and finally picks out a thumping forehand down the line to close out the game.
Plenty of intrigue, cut and thrust to get your teeth into in those opening exchanges though...
Djokovic to serve
*Djokovic 0-0 Nadal
Both players pumping down the practice serves.
Neither has banished their entourages to the gods like Andy Murray did last night - Rafael Nadal's other half is down courtside alongside his sister.
Novak Djokovic will get first use of the service fuzz.
Nadal could return to top of game
Andrew Castle
BBC Sport
"This tournament is the first time in a long time that it feels like Rafael Nadal could climb back to the very top of the game."
Rafa back to where he should be
Tim Henman
Former British number one on BBC One
"In the last five or six weeks Rafa has got his game back to where it should be.
"He has been more aggressive and flattened his backhand out. He has nothing to lose and lots to gain today."
Post update
Djokovic v Nadal
Knock-up underway
Djokovic v Nadal
The stadium emcee raps his vocal cords around Rafa Nadal's vowels with relish as he announces the players' arrival on court.
The players unwrap their bats in near darkness as the house lights are kept off and the dance music keeps pumping.
It is like a sweaty spinning class at the O2.
To the net. Rafa bouncing, Novak stretching hamstrings. Umpire Mohamed Lahyani issues final instructions and competition winner 'Sharon' flips the coin and poses for photos.
Knock-up under way.
BBC coverage
Djokovic v Nadal
BBC One
Quick, everyone over to BBC One.
A lovely sepia start to the television coverage, full of clenched fists and grimaces from past meetings between Novak Djokovic and Rafael Nadal.
Down to business and Andrew Castle has ticked the "era-defining rivalry" soundbite off your bingo card.
Get involved via the play icon at the top of this page.
Nadal the one to watch in 2016
Piers Newbery
BBC Sport at the O2 Arena in London
"He showed what's to come in 2016, in my opinion.With some time to practise and reassess his game, he's going to be tough to beat in Australia. I believe that." Nadal's resurgence has not gone unnoticed by the rest of the field and Roger Federer is predicting big things from his great rival next year. Nadal himself has been in an unusually cheerful mood all week in London and is planning an intensive off-season, even by his standards. "I don't deserve a vacation," he joked. We think.
Swiss role
This is the day's first semi-final of course.
The evening session, appropriately enough for an arena set in the heart of London's financial engineroom, is an all Swiss affair.
Tennis' Dorian Gray Roger Federer takes on the majestic backhand that comes attached to Stan Wawrinka.
The first serve crashes down in that one shortly after 20:00 GMT.
Skill and crossed racquets
Piers Newbery
BBC Sport at the O2 Arena in London
It's an icily cold day on the North Greenwich Peninsula, a place where the corpses of pirates were once hung in cages to warn off others. Now they play tennis in a giant tent, which is a much better idea. Semi-finals day has provided real drama here over the years, from Murray and Nadal's epic in 2010 to Federer and Wawrinka's falling out last year. There are 17,000 spectators now making their way through security fully expecting another thriller this afternoon.
Head to head
Djokovic v Nadal
Since then Rafa has cut his hair, Novak has cut out gluten and one of the great rivalries in modern tennis has blossomed.
There have been 45 matches between the pair, with Nadal narrowly ahead 23 to 22 on the countback.
And what gems there are in that long list.
Rafael Nadal winning out in a five-set epic in the French Open semi-finals in 2013.
Novak Djokovic coming through in five hours and 53 minutes in a pulsating Australian Open final in 2012.
What awaits us this afternoon as they go head to head for a place in the World Tour Finals?
Back in the day
Djokovic v Nadal
The first Tour meeting between Novak Djokovic and Rafael Nadal was back in June 2006 when Tim Henman and Greg Rusedski were still knocking around in the first week of the French Open.
Nadal and Djokovic met in the quarter-finals at Roland Garros with the Spaniard prevailing via a retirement when 6-4 6-4 up.
It did not feel like the start of something.