Kei Nishikori serving to stay in the set.

A solid first serve to steady the nerves and win the first point.

A forehand down the line singes the top of the tape and sets up 40-15 and a rifle-crack of a serve out wide beats Federer's plunging racquet.

Image source, Reuters

Kei Nishikori with the shot of the match so far as he wins a dinky net exchange with a cute backhand across court from virtually over his own shoulder.

A nice one for the highlights reel, but it does not make any impression on the scoreboard.

Image source, Getty Images

All very well sitting in the slipstream. Can Kei Nishikori do the front running?

Two service holds away from the first set, the Japanese frames a forehand horribly to hand Roger the first point.

Federer sniffs out the whitewash out wide - running the legs out of Nishikori - to take the game to 30-30.

Early in the match, but already this feels like the full Swiss inquisition as Federer mixes it up in a lengthy rally and milks Nishikori for the mistake to buy his way to deuce.

Advantage Federer. Nishikori overreaches with his first serve. Scurry and squeak as the pair's shoes chase down the ball in a lengthy rally on break point.

Nothing wrong with Nishikori's footwork. The racquet is the problem though and he prods wide to allow Federer back to parity.

Now then. Kei Nishikori, zipping about the court like a battery bunny, has 0-30 to look at.

A wicked angled backhand from well out in the wings lands for a winner across court past a stranded Federer for 15-40.

And, whip-crack ka-blammo, there it is. Nishikori, seeing the ball like a beach ball, is a break up.

Image source, AP

Kei Nishikori with the wind in his sails now.

The Japanese rattles along to 40-0 and, with a comfortable cushion, chooses to stand and deliver from the baseline.

A crushed forehand just wide and a Federer backhand right up onto the baseline and suddenly Nishikori is looking a lot more fidgety at 40-30.

He makes it over the line though and is back to even stevens on the scoreboard.

Roger Federer never really recovers from a slow start to the game after folding out of its first two points.

Down 0-30, the Swiss has an ace in his pocket to minimise the damage.

Nishikori clambers up to 15-40, but looks a little choky as he coughs up a couple of unforced errors to allow two break points to slip away.

A third from deuce. And this time a dipping return, right on Roger's sponsored tootsies, proves the ticket.

Into the net on the half-volley from Federer and Nishikori has the break back.

Image source, AP

Kei Nishikori in a spot of bother at 15-30 down and with his serve as twitchy as a freshly landed trout.

A supreme forehand down the line reels the Swiss back in to 30-30.

But Federer forces the match's first break point at 30-40.

Back and forth they duel. Cat and mouse, nip and tuck, as the two players trade cautious cagey backhands, fizzing with spin.

Finally Federer applies an overdose of sauce to the ball and Nishikori overcooks wide in reply.

First blood to the Fed.

80s Time Tunnel: , externalAll-time favourite, Stefan Edberg. Im a serve and volley fiend! #80sTennis, external#BBCTennis, external@bbctennis, external

Sharon Bailey, external: #bbctennis, external Andre Agassi made tennis exciting to watch. All-time favourite although Rafa pretty cool too.

Andrew James, external: @bbctennis, external It has to be Hewitt such a fighter and true icon. #bbctennis, external

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Roger Federer is just playing for kicks. That is the theory...

He is already through to the last four, regardless of today's result, but there is no let-up in the 24-carat quality at the moment.

He upbraids himself for a sloppy double fault, celebrates winning a point at 30-30 with a clenched fist and sees the game out with a drop-shot full of foxy cunning.

He is fully engaged with this 'dead rubber'.

Kei Nishikori is slicing and dicing off the backhand, trying to throw a spanner in the Federer game and break up the great man's rhythm.

A nice raid to the net, but that volley is buried south. Federer nudges in to 40-30. But the world number three cuffs a backhand long and Nishikori is on the board.

Image source, EPA

Roger Federer eases out of the blocks, serve purring like a Rolls Royce with some precision deliveries out to either wing.

40-0 in a twinkle. Nishikori gets his eye in as he nibbles back a couple of points, but that is all he is having.

Image source, Getty Images

BBC TV presenter Sue Barker has been talking about Roger Federer's popularity.

"You've just seen how popular he is and he gets that sort of reception all over the world," she said.

"There are so many Roger Federer fans here. It's the way he plays, the manner he plays."

But Federer aside, who is your all-time tennis favourite? Sue cheekily plumped for colleague Tim Henman, while he sided with Bjorn Borg.

Get involved via #bbctennis, externalor the BBC Sport Facebook, external page.

It is usually a slick show at the O2.

But this time, the simple matter of the coin toss gets away from the umpire.

A competition winner flips the small change. The coin disappears into the bottom of the net. The umpire says that Nishikori has called right. Roger has a curious peer downwards, sniffing a mistake.

Nishikori has got it right. 

But Roger is so competitive, he might call for a stewards' enquiry into whether it was heads or tails. 

Image source, Getty Images

More dry ice than a Boyzone concert, more tannoy decibels than an X Factor live show and the two players are out on court.

Kei Nishikori lays out his towel on his courtside seat neat and tidy.

Roger Federer, looking a little stubbly, strides out in his wake looking stern and focused.

Image source, Getty Images

The head-to-head record between these two is tighter than a racquet string.

Federer leads by the odd victory in five.

His Swissness has won their last two meetings, including a 6-2 6-3 success in their round-robin stage of this tournament. 

Piers Newbery
BBC Sport at the O2 Arena in London

Image source, Getty

The players have been making their way to the O2 by various means this week, with most taking a 40-minute boat ride along the Thames from The Savoy - the official hotel. Jamie Murray uses his local knowledge to get an early Tube from home, brother Andy has nipped home to Surrey some nights and driven himself in. Federer, meanwhile, is not staying with the other players and almost missed kick-off time against Djokovic on Tuesday when his journey to the O2 took 2½ hours. Maybe he should borrow Jamie's Oyster card.

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Piers Newbery
BBC Sport at the O2 Arena in London

It's a soggy afternoon by the Thames but typically roasting inside the O2 Arena, where the snug temperature has already caused more than one fan to nod off during the quieter moments. That should not be an issue today as we get to the business end of the group stage, with three men battling to join Federer in the semis. Djokovic is best placed, Berdych the outsider, with Nishikori the potential surprise package. I'll leave the specific scenarios to you...

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Roger Federer is already safe and snug in the last four of the World Tour finals after inflicting Novak Djokovic's first defeat in three months on Tuesday.

Can Nishikori join him in the semi-finals?

Well, chances are on the skinny side.

The 25-year-old must beat Federer and hope that Tomas Berdych upsets Novak Djokovic this evening.

Here is Piers with a full run-down on the possible survivors from Group Stan Smith.

Image source, Getty Images

Getting Kei Nishikori up into the world's top 45 - a new national high for a men's singles player - became something of an obsession for Japanese tennis five years back.

In the summer of 2011 he teetered tantalisingly at 46th on the rankings ladder. And then plummeted to 60th. But finally achieved his goal in October of that year.

And he has not looked back since. Last year, he made his first Grand Slam final appearance - losing to  Marin Cilic at the US Open - and made the semi-finals on his first appearance at the the O2.