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Roger Federer: "It was extremely difficult. It was a good match, with more breaks than we are used to. Kei is a very good returner, especially on the second serve. It was a very physical match, but I enjoyed it.

"Kei was staying aggressive and going for his shots and that makes it hard. I had to hope that he would slip.

"Rafa, Andy or Stan - it is gong to be rough either way in the semi-finals! Regardless, I'm looking forward to it."

Kei Nishikori smacks away a running volley to sniff an escape at 40-30.

But what a time for his first double fault of the third set to arrive. 

We are to deuce.

Scratch that. Match point Federer.

Roger Federer swipes a magnificent backhand winner across court for 15-30. 

He is two points away.

Elaine Heyworth: All time favourite, bar Roger - Ivan Llendl. Absolutely loved him.

Dez Kamara: Federer is the GOAT. Respect to Pete Sampras and others but bow to King Roger. Always cool as a cucumber.

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Roger Federer feathers the pedal once again. He takes the game to love.

We are in tight-rope territory. One slip for either man and it is likely to be curtains.

Kei Nishikori holds to love, wrapping up the final point of the game with a nice piece of double-bluff as he dummies to return a drop-shot with similar before accelerating the racquet head through the ball.

The best match of the Tour Finals so far this year? No doubt about it.

Nishikori back to 4-4 from 4-1 down for the second set in succession.

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Roger Federer has somehow ended up in a dogfight here.

With nothing riding on this match for him, a less proud man might have let a stubborn Nishikori have his way and save their energy for the semi-finals.

But that man would not have 17 Grand Slam titles to their name.

Mammoth rallies, tricky angles, lactic loading through both players, and Nishikori finally wins the armwrestle to take the decider back on serve.  

Tim Henman
Former British number one on BBC Two

You see some rallies and think if this was a boxing match, the referee should step in - but Nishikori keeps coming back for more. He's such a good competitor.

The SABR - 'sneak attack by Roger' - gets dusted off for the first time as Federer darts in mid-ball toss to get up close and personal with a Nishikori second serve.

It is so crazy it just keeps working, but it is not enough for Federer to knock Nishikori a double break down.

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Australia rugby union captain and coach Stephen Moore and Michael Cheika - on Barbarians duty this weekend - soak up some O2 atmosphere.

Out on court, Federer is weaving some of the old magic. A terrific pick-up half-volley, loaded with top-spin, lands to consolidate the break.

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A real slug-fest of a game.

Kei Nishikori hangs very tough in a extended rally at 15-30 down, matching Federer's probing angles with relentless chase and stretch.

Federer's language gets a little less than statesman-like as he coughs up an error to allow Nishikori back to 40-30.

Deuce.

Oh brilliant. Nishikori with a brilliant one-hand backhand punch down the line after guessing right on where Roger's attempted winner was heading.

But there is no escape. Federer crunches a second-serve return, Nishikori cannot prise it off the floor and Federer bellows a greeting to the break.

Paul Olden: For me it has to be Jan Kodes, I was 12 when he won Wimbledon in the year of the player boycott in 1973, I watched it on the BBC and my love affair with tennis began and still endures!

Delphine Gaudeix: André Agassi was always great to watch, Tsonga is always interesting to see but Roger all the way.

Stephan Wittendorp: Justine Henin. Best one-handed backhand ever.  

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With his form gone walkabout, Roger Federer survives on street-fighting instinct alone.

He rattles off five points in a row to redeem himself from 0-40 down.

There is still some grit amid all that stardust glitter. Federer clambers back to deuce.

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Roger Federer backed up to 0-40. He has come off the boil big time in the last half hour or so. Very ordinary tennis...

Shadow-boxing under the O2 lights.

Kei Nishikori delivers his service game safely enough, under minimal pressure from the other end.

The Japanese must win this match and hope for an upset from Tomas Berdych against Novak Djokovic tonight to stay in the tournament.

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Roger Federer is on power-saving mode, keeping his batteries charged for the final stages of these World Tour finals.

A mediocre half-volley slap wide from his baseline - feet rooted and in the wrong position - allows Nishikori a couple of points in the opening game of the third set.

This is not the Djokovic conqueror we saw earlier this week. And, to be fair, it doesn't need to be in a match that is effectively a dead rubber for the Swiss.