Image source, AFP

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.

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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.

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."

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.

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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.

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", external - hears something she doesn't like.

Expect the snappers' lenses to swing to the players' box sharpish. 

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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."

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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", external

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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...

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.

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

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?

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."

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... 

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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.

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.

Piers Newbery
BBC Sport at the O2 Arena in London

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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'.

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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.

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.

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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.