Stan Wawrinka beats Marin Cilic 6-3 4-6 6-3
Swiss had secured semi-final spot by winning three games
Wawrinka now leads head-to-head 8-2
Two groups of four in round-robin format
Russell Fuller
BBC tennis correspondent on Radio 5 live sports extra
"Cilic has won his first set of the World Tour Finals, and he's won a few friends here at the O2 Arena as we're going to see a deciding set for only the second time this week."
David Law
BBC Radio 5 live sports extra commentator
"There's a lot of money riding on this third set - whoever wins it gets an extra $155,000 [£98,900]. But there's a feeling that these two are putting on a show now, they really want to entertain this crowd."
Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
"Wawrinka probably wouldn't have wanted to play a third set this evening, but he's got to regroup as he's going to have to play very, very well against Roger tomorrow evening."
Image source, Getty ImagesWawrinka has dazzled with the occasional forehand winner, but has not found his rhythm in this set. A dreamy Cilic return sends Wawrinka chasing fresh air, the crowd sense the Croat could be about to pounce at 0-30 - and so he does. Wawrinka going off radar and we're into a third set for just the second time in this tournament.
BBC Radio 5 live sports extra's commentators have been asking you to construct your perfect tennis player:
Tim Oliver:, external Whichever players forehand/volley/backhand etc you choose, surely the perfect tennis player must need the stamina of John Isner?
Cilic's serve is hotting up. He sends an ace to the wings, nudging to within a point of another hold, and secures the game with a far from convincing overhead. Convincing or not, Cilic is in the fight.
Dave of Acton:, external ATP Tour Finals need to stay in London. Nowhere else could pull in such passionate crowds for every session, even dead rubbers!
Wawrinka approaches the net on the first point, varying play nicely, sending Cilic towards the tramlines before seeing him off with a volley towards the opposite direction. The Swiss has his rival pinned on the baseline and looks confident at 30-15. Cilic forced to swing in hope and the gamble doesn't pay off and Wawrinka sends another sizzling ace down the 'T' to finish off the game.
Russell Fuller
BBC tennis correspondent on Radio 5 live sports extra
"Another ace... both men are looking secure on serve in this second set."
Cat-like reflexes from Wawrinka as he powers a crosscourt return into the corner - Cilic wouldn't have caught that had he started chasing it on Monday, but it's the only chance the Croat, adding another ace to the stats, gives the Australian Open as the server dominates once more.
Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
Image source, Getty Images"We're seeing why Wawrinka had that tournament success in the first part of the year. He's got so many tools in his game, he has variety and we've seen him skilful and confident around the net."
Oohs from the crowd as Wawrinka pings a 92mph forehand beyond Cilic, but Cilic works Wawrinka this way and that on the next point, before the Swiss goes long, way long, too long. An ace from Wawrinka, his third of the match, followed by another brings the game to a rapid conclusion.
Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
"The way the crowds seem to enjoy having the Tour Finals here - sometimes it's about not changing a winning formula."
Russell Fuller adds: "I've heard Abu Dhabi is putting in a big bid to stage the finals, but I'm not sure it would be able to replicate the atmosphere we have in London. They'll make a final decision in March or April, but want to sign a deal somewhere for five years."
BBC Radio 5 live sports extra's commentators have been asking you to construct your perfect tennis player:
FreeTiberio:, external Forehand Rafael Nadal; backhand Stan Wawrinka; serve, slice & net play Roger Federer; return & mobility Novak Djokovic.
Nuren Ahmed:, external Serve: Pete Sampras. Forehand: Nadal. Backhand: Wawrinka. Return: Agassi/Djokovic. Net play: Stepanek. Movement: Federer.
Big hitting from the big Croat. Wawrinka attempts to intimidate his rival by sprinting towards the tape, but Cilic simply passes him at the net, wrapping up another trouble-free hold to 15. We're not going to three sets, are we? Surely not?
Image source, Getty ImagesThe camera goes in search of celebrities and finds Scary Spice and Ronnie O'Sullivan (sitting separately). Whoops and whistles from the spectators as Wawrinka outfoxes Cilic with a feather-soft touch at the net which brings up 40-0 on the scoreboard. And the Swiss wraps up the hold with a crisp ace down the middle. A hold to 15. Both players prepared to go toe-to-toe in the early stages of this set.
Image source, Getty ImagesCilic wipes away the sweat from his brow as he returns to his chair for the changeover, but he exerted little energy to secure that game, which was one of his more comfortable holds. A hold to 15 and on we go.
BBC Radio 5 live sports extra's commentators have been asking you to construct your perfect tennis player:
Jack Sterne:, external Serve: Ivo Karlovic. Forehand: Nadal. Backhand: Djokovic. Movement: Murray. Return: Ferrer. Tactical guise: Federer.
Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
"Longer rallies are good on the eye for spectators - slower courts give the opportunity to get the return in. But tennis purists may welcome more variety in the game."
Cilic is out of the tournament but isn't out of the match. He still has a chance in this game, too, as Wawrinka chucks in a double fault to let his opponent level at 30-30. Wawrinka, though, comes out on top of another baseline battle, the Swiss with oodles of time to thread a forehand winner down the line. And it's an ugly end from Cilic, his return turning into a cover drive. It's even stevens in this second set.