Wawrinka is still signing autographs and posing for pictures but it's time to end our live text commentary and prepare ourselves for a semi-final bonanza. On Saturday afternoon, world number one Novak Djokovic will play the man who beat him at this year's US Open semi-final, and in the evening session friends will become enemies as Wawrinka takes on Roger Fedeer for one night only (well, for the last time this year at least). Thanks for your tweets and texts. Bye for now.
BBC coverage
Tomorrow, you can watch coverage of the first semi-final between Novak Djokovic and Kei Nishikori on BBC Two and the BBC Sport website from 14:00 GMT.
BBC Two Scotland viewers will need to press their Red Button just before 15:00 GMT to continue watching the tennis, as BBC Two Scotland switches to coverage of the Scottish National Party's annual conference at that point.
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Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
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"I hope Wawrinka gets a fair amount of the support tomorrow, as we know Roger is so well loved. I think it'll be Novak and Roger in the final - but let's hope we get a third-set match at some point."
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Wawrinka wipes his sweaty visage before signing a camera lens with a yellow pen. Two group victories to set up an all-Swiss semi-final.
"It was a tough match, it's never easy to play against Marin, he's been having an amazing year," says Wawrinka. "Today it was not easy, trying to be aggressive. I was trying to serve and volley. To win it's important for me."
Asked about the prospect of playing his compatriot Federer, Wawrinka says: "I hope I can get a little bit of support because I know people love him here. It's going to be tough for me, but I need to trust myself and go for it."
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Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
"Obviously a shame to finish with a double fault, but Cilic is getting a warm round of applause. His first two matches were poor, but he picked it up tonight. You can tell that even though Wawrinka had already qualified, it meant a lot to him to win tonight."
Game, set and match
*Wawrinka 6-3 4-6 6-3 Cilic
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Oh la la! Is there a more beautiful sight in tennis than a backhand winner from Wawrinka? That shot takes the Swiss to 30-15, and an anxious Cilic goes long on the next point. Two match point Wawrinka. Cilic saves one but double-faults on the next to surrender with a whimper.
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Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
"Wawrinka took such a long time to establish himself as one of the world's top players, he will cherish every moment on a stage like this."
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Wawrinka 6-3 4-6 5-3 Cilic*
Wawrinka isn't in a mood to let his lead slip. The Swiss collects three points, finishing off a love hold with a forehand onto the line. We won't be keeping you for much longer.
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BBC Radio 5 live sports extra's commentators have been asking you to construct your perfect tennis player:
James Alex: Nadal/Del Potro forehand, Wawrinka backhand and the champion mentality of Federer makes the complete tennis supreme. Federer to win a Slam in 2015 and Nishikori/Dimitrov to win a maiden Grand Slam.
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Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
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"Three unforced errors in a row there from Cilic, all three on the backhand side which is his favourite shot, so he'll be sitting there at the end change wondering what's going on."
Wawrinka breaks
*Wawrinka 6-3 4-6 4-3 Cilic
At 15-15, Cilic launches a serve out to the wings, sending Wawrinka lunging at fresh air. The Croat will perhaps regret not finding his serving groove earlier in this tournament. From 30-30 to 40-30 as Wawrinka thumps an impatient forehand beyond the baseline. The third seed is Zen-like on the next point, adding some spin and slice and patiently waiting for Cilic to crumble, which he does. The Croat's backhand malfunctioning, going long to present Wawrinka with a break point - and long once again. "Come on," Wawrinka growls.
Double Swiss
Wawrinka 6-3 4-6 3-3 Cilic*
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Plenty of Swiss fans inside the arena - no doubt looking forward to a Federer-Wawrinka semi-final tomorrow evening. Whoops from the Swiss contingent for their man as he serves and volleys to a 30-0 lead. Cilic nicks one point, but Wawrinka has him on the backfoot on the next, leaving Cilic with little option but to attempt a lob, which bounces long this time. Wawrinka restores parity, holding to 15.
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Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
"I'm really pleased Cilic has managed to turn this around. This hasn't been a great tournament for him but no matter what happens now, he'll be finishing on a high and showing what he's capable of."
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*Wawrinka 6-3 4-6 2-3 Cilic
Cilic playing with freedom and that means sometimes he finds the line, and sometimes he doesn't. In this game, though, the Croat's forehand is on fire, helping him to a speedy love hold. An enjoyable contest.
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Wawrinka 6-3 4-6 2-2 Cilic*
Federer was probably reading bedtime stories at the beginning of this match, but if he's tuned in for the third set he won't be overly worried by what he is seeing. Wawrinka blowing hot and cold, as is Cilic. An ugly overhead from Croat when the court was at his mercy - but a few points later he drags Wawrinka to deuce thanks to a lob which drops perfectly on the baseline. Deuce. Advantage. Deuce. Advantage. Wawrinka directing a lethal ace out wide. Cilic had no chance.
Wawrinka breaks
*Wawrinka 6-3 4-6 1-2 Cilic
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Cilic swinging freely and without fear, while his opponent is thinking about ifs, buts and maybes. But, just as we are about to talk about another comfortable hold, Cilic produces a series of unforced errors - a double fault forcing the match to deuce. The Croat fails to negotiate the barrier and Wawrinka has break point. A disappointing end for Cilic as he shoots an adrenalized forehand into the tramlines. Wawrinka celebrates with a stentorian roar.
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BBC Radio 5 live sports extra's commentators have been asking you to construct your perfect tennis player:
Muhammad Tayyab: Sampras's serve, Federer's artistry, Nadal's relentlessness, Murray's retrieving skills and Djokovic's flexibility. Federer with Djokovic's backhand would sweep aside every tennis player including Nadal.
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We've also been asking who you can see breaking into the top level of world tennis in 2015:
Chris Martins: Dusan Lajovic could be another good Serbian player to come through the ranks after some impressive performances!
Cilic
Wawrinka 6-3 4-6 0-2 Cilic*
Wawrinka balloons a forehand beyond the baseline and shakes his head before looking towards his camp. The Swiss seeing red mist and a double fault doesn't improve his mood. Two break points Cilic. A nervy baseline slug-fest ensues, nervous aahs from the spectators as the ball tickles the net chord but the rally carries on... before Wawrinka produces a sublime backhand to bring the rally to a brilliant end. But on the next point, the Swiss messes it up, goes for the winner and gets it all wrong. Cilic breaks
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*Wawrinka 6-3 4-6 0-1 Cilic
The Swiss is through to the semi-finals but will want to find his swing again before meeting Federer tomorrow. Unforced errors gift Wawrinka two break points. Is the Swiss hot or has Cilic gone cold again? A bit of both. The Croat serves his way out of danger and from 15-40 down he wriggles free.
"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."
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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."
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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."
Game and second set
Wawrinka 6-3 4-6 Cilic*
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Wawrinka 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.
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BBC Radio 5 live sports extra's commentators have been asking you to construct your perfect tennis player:
Tim Oliver: Whichever players forehand/volley/backhand etc you choose, surely the perfect tennis player must need the stamina of John Isner?
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*Wawrinka 6-3 4-5 Cilic
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.
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Dave of Acton: ATP Tour Finals need to stay in London. Nowhere else could pull in such passionate crowds for every session, even dead rubbers!
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Wawrinka 6-3 4-4 Cilic*
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.
"Another ace... both men are looking secure on serve in this second set."
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*Wawrinka 6-3 3-4 Cilic
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.
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Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
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"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."
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Wawrinka 6-3 3-3 Cilic*
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.
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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."
Get involved via #bbctennis
BBC Radio 5 live sports extra's commentators have been asking you to construct your perfect tennis player:
FreeTiberio: Forehand Rafael Nadal; backhand Stan Wawrinka; serve, slice & net play Roger Federer; return & mobility Novak Djokovic.
Nuren Ahmed: Serve: Pete Sampras. Forehand: Nadal. Backhand: Wawrinka. Return: Agassi/Djokovic. Net play: Stepanek. Movement: Federer.
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*Wawrinka 6-3 2-3 Cilic
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?
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Wawrinka 6-3 2-2 Cilic*
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The 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.
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*Wawrinka 6-3 1-2 Cilic
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Cilic 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.
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BBC Radio 5 live sports extra's commentators have been asking you to construct your perfect tennis player:
Jack Sterne: Serve: Ivo Karlovic. Forehand: Nadal. Backhand: Djokovic. Movement: Murray. Return: Ferrer. Tactical guise: Federer.
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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."
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Wawrinka 6-3 1-1 Cilic*
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.
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*Wawrinka 6-3 0-1 Cilic
Cilic hauls himself back from 0-30 to 30-30 with the help of an ace down the 'T' - only his fourth of the tournament. Another howitzer from the Croat? Wawrinka doesn't think so... der-dum... der-dum... and neither does Hawk-Eye. Second serve finds the target and Cilic collects another point. In the end, it is a straightforward hold. Ivanisevic applauds on the sidelines. Is the US Open champion preparing a fightback?
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We've been asking who you can see breaking into the top level of world tennis:
Bob in Spain: Watch out for Borna Coric as a future top player. Already ranked 89 in the world and turning 18 today.
John McEnerney: If Rod Laver, Pat Cash, Hewitt & The Woodies all gave Tomic a good clip around the ear & kick in the pants he could rule tennis!
Kayley Jay: David Goffin should do well - hopefully he won't get injured again! I'd like to see him make another good run. Top 10 next year?
Game and first set
Wawrinka 6-3 Cilic*
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Easy peasy. 'Set Point' flashes up on the advertising boards around the court as Wawrinka advances to a 40-0 lead. Cute from the Swiss, who drags Cilic towards the tape with a drop shot and simply watches his opponent push his volley into the tramlines.
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Piers Newbery
BBC Sport at the O2 Arena in London
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"The tennis might have been fairly underwhelming so far but the staging has once again impressed, and another 17,000 spectators are packed into the O2 this evening. The venue has one year left before the rival cities can make a bid to take the tournament away - something that would surprise John McEnroe.
"'You're asking someone who grew up in New York, who used to play what I considered one of the great events in Madison Square Garden,' said the American. 'They should never, ever have left there. Having said that, this has been clearly the most successful of all the six or seven locations that have been since we left Madison Square Garden. It would be hard-pressed for me to imagine not having it either here or there.'"
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*Wawrinka 5-3 Cilic
"Thank you," says the umpire in an attempt to quieten one or two distant cries from those in the vertigo-inducing seats. Wawrinka soon has the crowd purring with a forehand winner which fizzes by Cilic on the opening point. At 15-15, Cilic is undone by a Wawrinka lob. It was the Croat's poor approach play, rather than Wawrinka's brilliance, which killed him off on that point. Wawrinka not firing on all cylinders but it might be good enough to see off Cilic.
To deuce we go, but neither player wants to take the upper hand. It's itsy and it's bitsy. Cilic, though, comes through, Wawrinka putting a touch too much juice on his returns.
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David Law
BBC Radio 5 live sports extra commentator
"Coming into the foyer this evening, it was very busy as there were 17,000 people here for the tennis, and a lot of people rather alarmingly dressed for an evening at the local nightclub. They get a bit rowdy inside the O2 here too, the evening sessions are rather different to the afternoon when people are a little more polite."
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Wawrinka 5-2 Cilic*
A series of errors from Wawrinka allows Cilic to work his way back from 30-0 to 30-30. But the Croat goes on to net a return on the next point and follows that up by sending another beyond the baseline. A messy hold, but Wawrinka is set to nudge ahead in this match.
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We've been asking who you can see breaking into the top level of world tennis:
Max, Nottingham: Dimitrov has all the attributes of a future legend. All he needs is a physical change much like Murray went through around the same age. With Roger Rasheed training him, it's a matter of 'when' rather than 'if'. I'm excited!
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Keith Punter: US Open champion or not, Cilic has sadly been out of his depth this week.
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*Wawrinka 4-2 Cilic
For those interested, Cilic is wearing a cobalt blue t-shirt and daffodil yellow trainers. The US Open champion's hands are as delicate as a petal at the net, too, as he exchanges volleys with Wawrinka before responding to a lob with an overhead.
At 30-30, the game could swing either way, but Cilic floors Wawrinka with a booming serve and holds thanks to a textbook serve and volley. Nothing more to see here.
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Wawrinka 4-1 Cilic*
A double fault. An ace. A stonking single-handed backhand winner down the line. An iffy backhand into the tramlines. Wawrinka up and then down, or down and then up, in this game. Cilic forces the game to deuce, before Wawrinka sees off the challenge with a sweet serve and volley.
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Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
"Kei Nishikori is someone who can get close to the top players, and it will be interesting to see how Grigor Dimitrov develops - he's a lot like a young Roger Federer, he has a good coaching set-up and has a lot of hunger."
Who can you see breaking into the top level of world tennis? Text and tweet us with your thoughts.
Wawrinka breaks
*Wawrinka 3-1 Cilic
Gasps from the crowd as Wawrinka returns a Cilic serve with the speed of a Cobra's tongue. Brilliant from the Australian Open champion. He fetches and retrieves from the baseline, defending stoutly, before Cilic falters - presenting his opponent with three break points. The Croat saves one, but approaches the net and volleys into the tramlines on the next point. A break of serve, a third game in the bag, and Wawrinka is through to the semi-finals.
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Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
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"Wawrinka leapt onto that forehand and clubbed it down the line. Great tennis."
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Wawrinka 2-1 Cilic*
Solid serving once again from Wawrinka. A serve and volley eases him to 40-0. Cilic absorbs the punches on the next point, though, counter-attacking with another thunderous forehand down the line and Wawrinka's response is to overcook a forehand. Again, Cilic nicks just the one point on the Wawrinka serve, the Swiss ending the match with a picture-perfect forehand winner which grazes the tramline.
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Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
"This is a terrific tournament, it's more like the Davis Cup and much better for the spectators."
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*Wawrinka 1-1 Cilic
Both players unleashing the big shots early. Cilic with a forehand down the line, Wawrinka challenges the call, but replays show the Croat kissed the line by 0.000001mm. Close. To 40-15 we go and it's a routine hold for the US Open champion. This could actually be a contest.
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David Law
BBC Radio 5 live sports extra commentator
"If nothing else tonight, this crowd deserves to see the best of Marin Cilic - it deserves to see the US Open champion playing like the US Open champion."
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Wawrinka 1-0 Cilic*
Wawrinka sprints towards the back of the court to start this match. Eager. The Swiss made a fabulous start to this tournament, securing a sensational 6-1 6-1 victory over Berdych. Then he came up against Novak Djokovic and lost 6-3 6-0, but everyone seems to lose to the world number one these days so it should have no bearing on the outcome of this contest.
Two easy points for the Australian Open champion before he notches a third with a fabulous forehand winner into the corner. Wawrinka's single-handed backhand falters, gifting Cilic his first point of the game, but there's no danger of the Croat overhauling Wawrinka from here. Game Wawrinka. Two more and the Swiss is through to the last four.
Bye Bye Tomas
There'll be no more of Tomas Berdych at the 02 this year as two defeats in Group A means the Czech it outta here. The sixth seed has tweeted: "I had great year 2014. Thanx to my team, fans for your fantastic support in good and bad times!!! With ❤️ Tomas"
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Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
"Like most people who know Andy Murray well, I felt for him last night. But it's been a terrific end of year for Andy, to win that number of matches in the last five or six weeks shows you how far he's come. It's taken its toll on him, but it sets him up to have a good think in pre-season on what he needs to improve on. He does it most years - he goes away, comes back a better player, and he can look forward to 2015 now."
Spot the ex-Wimbledon champion
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To photobomb or not photobomb? It's a modern-day dilemma. I guess if you're a former Wimbledon champion you can get away with it. Marin Cilic's coach Goran Ivanisevic gets in on the act while the Tour Finals judges try to take a group photo.
Cilic the social climber
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It's been an incredible year for US Open champion Cilic but he hasn't been at his best in north Greenwich. The Croat has lost his two group matches, suffering a 6-3 6-1 defeat by Tomas Berdych and a 6-1 6-1 loss to Novak Djokovic. Incredibly, he has had his serve broken 10 times during this tournament. Will Wawrinka make him suffer some more before the season ends?
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Wawrinka and Cilic are at the net for the coin toss, Wawrinka continuing with those lunges. Never has a man's muscles been so prepared for battle. Anyway... it's Wawrinka who will serve first after the US Open champion decides to receive.
"Tomorrow evening's semi-final will be Roger Federer against, probably, Stan Wawrinka. There have been so many one-sided matches this week, that we should reserve judgement at least for the first 10 minutes - as Wawrinka could go through if he starts well."
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Last-minute stretching for Wawrinka, who is lunging in the corridor before the players are called to make their way to the court.
Cilic leads the pair out and, as he approaches the arena, he can probably hear the crowd clap along to the sound of the beating heart which is booming out of the sound system.
Those permutations
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Roger Federer awaits in the second semi-final. But, in an attempt to clear a few things up, here is who needs to do what:
If Wawrinka wins - then he qualifies for the semi-finals.
If Cilic wins in two sets - the runner-up in the group is determined by the percentage of games won.
If Cilic wins in three sets - Wawrinka qualifies for the semi-finals.
All Stan needs to do is to take this match to a third set. In fact, the percentages mean that if Cilic wins in two sets but Wawrinka wins three or more games along the way, Wawrinka goes through. Confused? No? Good.
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Wawrinka is within touching distance of the semi-finals. He's so so close. The Australian Open champion merely needs to win three games against Cilic to set up an all-Swiss semi-final with Roger Federer, although had Wawrinka watched his friend's 6-0 6-1 thrashing of Andy Murray last night he may prefer to enjoy London's Christmas markets on Saturday rather than play a compatriot who is in eye-wateringly good form.
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It's time to start the final countdown for the final showdown. After 11 matches, 23 sets and 187 games, the final group match is upon us.
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Are we too late for fireworks or too early? Stanislas Wawrinka versus Marin Cilic isn't ordinarily a match which would light up London's skies, but in a few hours the semi-final line-up for the ATP World Tour Finals will be decided and that is reason enough for pyrotechnics if you ask me. And it's Friday.
Live Reporting
Aimee Lewis
All times stated are UK
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Getty ImagesCopyright: Getty Images - If Wawrinka wins - then he qualifies for the semi-finals.
- If Cilic wins in two sets - the runner-up in the group is determined by the percentage of games won.
- If Cilic wins in three sets - Wawrinka qualifies for the semi-finals.
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Latest PostPost update
Wawrinka is still signing autographs and posing for pictures but it's time to end our live text commentary and prepare ourselves for a semi-final bonanza. On Saturday afternoon, world number one Novak Djokovic will play the man who beat him at this year's US Open semi-final, and in the evening session friends will become enemies as Wawrinka takes on Roger Fedeer for one night only (well, for the last time this year at least). Thanks for your tweets and texts. Bye for now.
BBC coverage
Tomorrow, you can watch coverage of the first semi-final between Novak Djokovic and Kei Nishikori on BBC Two and the BBC Sport website from 14:00 GMT.
BBC Two Scotland viewers will need to press their Red Button just before 15:00 GMT to continue watching the tennis, as BBC Two Scotland switches to coverage of the Scottish National Party's annual conference at that point.
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Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
"I hope Wawrinka gets a fair amount of the support tomorrow, as we know Roger is so well loved. I think it'll be Novak and Roger in the final - but let's hope we get a third-set match at some point."
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Wawrinka wipes his sweaty visage before signing a camera lens with a yellow pen. Two group victories to set up an all-Swiss semi-final.
"It was a tough match, it's never easy to play against Marin, he's been having an amazing year," says Wawrinka. "Today it was not easy, trying to be aggressive. I was trying to serve and volley. To win it's important for me."
Asked about the prospect of playing his compatriot Federer, Wawrinka says: "I hope I can get a little bit of support because I know people love him here. It's going to be tough for me, but I need to trust myself and go for it."
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Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
"Obviously a shame to finish with a double fault, but Cilic is getting a warm round of applause. His first two matches were poor, but he picked it up tonight. You can tell that even though Wawrinka had already qualified, it meant a lot to him to win tonight."
Game, set and match
*Wawrinka 6-3 4-6 6-3 Cilic
Oh la la! Is there a more beautiful sight in tennis than a backhand winner from Wawrinka? That shot takes the Swiss to 30-15, and an anxious Cilic goes long on the next point. Two match point Wawrinka. Cilic saves one but double-faults on the next to surrender with a whimper.
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Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
"Wawrinka took such a long time to establish himself as one of the world's top players, he will cherish every moment on a stage like this."
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Wawrinka 6-3 4-6 5-3 Cilic*
Wawrinka isn't in a mood to let his lead slip. The Swiss collects three points, finishing off a love hold with a forehand onto the line. We won't be keeping you for much longer.
Get involved via #bbctennis
BBC Radio 5 live sports extra's commentators have been asking you to construct your perfect tennis player:
James Alex: Nadal/Del Potro forehand, Wawrinka backhand and the champion mentality of Federer makes the complete tennis supreme. Federer to win a Slam in 2015 and Nishikori/Dimitrov to win a maiden Grand Slam.
Post update
Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
"Three unforced errors in a row there from Cilic, all three on the backhand side which is his favourite shot, so he'll be sitting there at the end change wondering what's going on."
Wawrinka breaks
*Wawrinka 6-3 4-6 4-3 Cilic
At 15-15, Cilic launches a serve out to the wings, sending Wawrinka lunging at fresh air. The Croat will perhaps regret not finding his serving groove earlier in this tournament. From 30-30 to 40-30 as Wawrinka thumps an impatient forehand beyond the baseline. The third seed is Zen-like on the next point, adding some spin and slice and patiently waiting for Cilic to crumble, which he does. The Croat's backhand malfunctioning, going long to present Wawrinka with a break point - and long once again. "Come on," Wawrinka growls.
Double Swiss
Wawrinka 6-3 4-6 3-3 Cilic*
Plenty of Swiss fans inside the arena - no doubt looking forward to a Federer-Wawrinka semi-final tomorrow evening. Whoops from the Swiss contingent for their man as he serves and volleys to a 30-0 lead. Cilic nicks one point, but Wawrinka has him on the backfoot on the next, leaving Cilic with little option but to attempt a lob, which bounces long this time. Wawrinka restores parity, holding to 15.
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Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
"I'm really pleased Cilic has managed to turn this around. This hasn't been a great tournament for him but no matter what happens now, he'll be finishing on a high and showing what he's capable of."
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*Wawrinka 6-3 4-6 2-3 Cilic
Cilic playing with freedom and that means sometimes he finds the line, and sometimes he doesn't. In this game, though, the Croat's forehand is on fire, helping him to a speedy love hold. An enjoyable contest.
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Wawrinka 6-3 4-6 2-2 Cilic*
Federer was probably reading bedtime stories at the beginning of this match, but if he's tuned in for the third set he won't be overly worried by what he is seeing. Wawrinka blowing hot and cold, as is Cilic. An ugly overhead from Croat when the court was at his mercy - but a few points later he drags Wawrinka to deuce thanks to a lob which drops perfectly on the baseline. Deuce. Advantage. Deuce. Advantage. Wawrinka directing a lethal ace out wide. Cilic had no chance.
Wawrinka breaks
*Wawrinka 6-3 4-6 1-2 Cilic
Cilic swinging freely and without fear, while his opponent is thinking about ifs, buts and maybes. But, just as we are about to talk about another comfortable hold, Cilic produces a series of unforced errors - a double fault forcing the match to deuce. The Croat fails to negotiate the barrier and Wawrinka has break point. A disappointing end for Cilic as he shoots an adrenalized forehand into the tramlines. Wawrinka celebrates with a stentorian roar.
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BBC Radio 5 live sports extra's commentators have been asking you to construct your perfect tennis player:
Muhammad Tayyab: Sampras's serve, Federer's artistry, Nadal's relentlessness, Murray's retrieving skills and Djokovic's flexibility. Federer with Djokovic's backhand would sweep aside every tennis player including Nadal.
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We've also been asking who you can see breaking into the top level of world tennis in 2015:
Chris Martins: Dusan Lajovic could be another good Serbian player to come through the ranks after some impressive performances!
Cilic
Wawrinka 6-3 4-6 0-2 Cilic*
Wawrinka balloons a forehand beyond the baseline and shakes his head before looking towards his camp. The Swiss seeing red mist and a double fault doesn't improve his mood. Two break points Cilic. A nervy baseline slug-fest ensues, nervous aahs from the spectators as the ball tickles the net chord but the rally carries on... before Wawrinka produces a sublime backhand to bring the rally to a brilliant end. But on the next point, the Swiss messes it up, goes for the winner and gets it all wrong. Cilic breaks
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*Wawrinka 6-3 4-6 0-1 Cilic
The Swiss is through to the semi-finals but will want to find his swing again before meeting Federer tomorrow. Unforced errors gift Wawrinka two break points. Is the Swiss hot or has Cilic gone cold again? A bit of both. The Croat serves his way out of danger and from 15-40 down he wriggles free.
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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."
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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."
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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."
Game and second set
Wawrinka 6-3 4-6 Cilic*
Wawrinka 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.
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BBC Radio 5 live sports extra's commentators have been asking you to construct your perfect tennis player:
Tim Oliver: Whichever players forehand/volley/backhand etc you choose, surely the perfect tennis player must need the stamina of John Isner?
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*Wawrinka 6-3 4-5 Cilic
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.
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Dave of Acton: ATP Tour Finals need to stay in London. Nowhere else could pull in such passionate crowds for every session, even dead rubbers!
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Wawrinka 6-3 4-4 Cilic*
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.
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Russell Fuller
BBC tennis correspondent on Radio 5 live sports extra
"Another ace... both men are looking secure on serve in this second set."
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*Wawrinka 6-3 3-4 Cilic
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.
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Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
"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."
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Wawrinka 6-3 3-3 Cilic*
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.
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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."
Get involved via #bbctennis
BBC Radio 5 live sports extra's commentators have been asking you to construct your perfect tennis player:
FreeTiberio: Forehand Rafael Nadal; backhand Stan Wawrinka; serve, slice & net play Roger Federer; return & mobility Novak Djokovic.
Nuren Ahmed: Serve: Pete Sampras. Forehand: Nadal. Backhand: Wawrinka. Return: Agassi/Djokovic. Net play: Stepanek. Movement: Federer.
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*Wawrinka 6-3 2-3 Cilic
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?
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Wawrinka 6-3 2-2 Cilic*
The 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.
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*Wawrinka 6-3 1-2 Cilic
Cilic 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.
Get involved via #bbctennis
BBC Radio 5 live sports extra's commentators have been asking you to construct your perfect tennis player:
Jack Sterne: Serve: Ivo Karlovic. Forehand: Nadal. Backhand: Djokovic. Movement: Murray. Return: Ferrer. Tactical guise: Federer.
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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."
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Wawrinka 6-3 1-1 Cilic*
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.
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*Wawrinka 6-3 0-1 Cilic
Cilic hauls himself back from 0-30 to 30-30 with the help of an ace down the 'T' - only his fourth of the tournament. Another howitzer from the Croat? Wawrinka doesn't think so... der-dum... der-dum... and neither does Hawk-Eye. Second serve finds the target and Cilic collects another point. In the end, it is a straightforward hold. Ivanisevic applauds on the sidelines. Is the US Open champion preparing a fightback?
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We've been asking who you can see breaking into the top level of world tennis:
Bob in Spain: Watch out for Borna Coric as a future top player. Already ranked 89 in the world and turning 18 today.
John McEnerney: If Rod Laver, Pat Cash, Hewitt & The Woodies all gave Tomic a good clip around the ear & kick in the pants he could rule tennis!
Kayley Jay: David Goffin should do well - hopefully he won't get injured again! I'd like to see him make another good run. Top 10 next year?
Game and first set
Wawrinka 6-3 Cilic*
Easy peasy. 'Set Point' flashes up on the advertising boards around the court as Wawrinka advances to a 40-0 lead. Cute from the Swiss, who drags Cilic towards the tape with a drop shot and simply watches his opponent push his volley into the tramlines.
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Piers Newbery
BBC Sport at the O2 Arena in London
"The tennis might have been fairly underwhelming so far but the staging has once again impressed, and another 17,000 spectators are packed into the O2 this evening. The venue has one year left before the rival cities can make a bid to take the tournament away - something that would surprise John McEnroe.
"'You're asking someone who grew up in New York, who used to play what I considered one of the great events in Madison Square Garden,' said the American. 'They should never, ever have left there. Having said that, this has been clearly the most successful of all the six or seven locations that have been since we left Madison Square Garden. It would be hard-pressed for me to imagine not having it either here or there.'"
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*Wawrinka 5-3 Cilic
"Thank you," says the umpire in an attempt to quieten one or two distant cries from those in the vertigo-inducing seats. Wawrinka soon has the crowd purring with a forehand winner which fizzes by Cilic on the opening point. At 15-15, Cilic is undone by a Wawrinka lob. It was the Croat's poor approach play, rather than Wawrinka's brilliance, which killed him off on that point. Wawrinka not firing on all cylinders but it might be good enough to see off Cilic.
To deuce we go, but neither player wants to take the upper hand. It's itsy and it's bitsy. Cilic, though, comes through, Wawrinka putting a touch too much juice on his returns.
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David Law
BBC Radio 5 live sports extra commentator
"Coming into the foyer this evening, it was very busy as there were 17,000 people here for the tennis, and a lot of people rather alarmingly dressed for an evening at the local nightclub. They get a bit rowdy inside the O2 here too, the evening sessions are rather different to the afternoon when people are a little more polite."
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Wawrinka 5-2 Cilic*
A series of errors from Wawrinka allows Cilic to work his way back from 30-0 to 30-30. But the Croat goes on to net a return on the next point and follows that up by sending another beyond the baseline. A messy hold, but Wawrinka is set to nudge ahead in this match.
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We've been asking who you can see breaking into the top level of world tennis:
Max, Nottingham: Dimitrov has all the attributes of a future legend. All he needs is a physical change much like Murray went through around the same age. With Roger Rasheed training him, it's a matter of 'when' rather than 'if'. I'm excited!
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Keith Punter: US Open champion or not, Cilic has sadly been out of his depth this week.
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*Wawrinka 4-2 Cilic
For those interested, Cilic is wearing a cobalt blue t-shirt and daffodil yellow trainers. The US Open champion's hands are as delicate as a petal at the net, too, as he exchanges volleys with Wawrinka before responding to a lob with an overhead.
At 30-30, the game could swing either way, but Cilic floors Wawrinka with a booming serve and holds thanks to a textbook serve and volley. Nothing more to see here.
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Wawrinka 4-1 Cilic*
A double fault. An ace. A stonking single-handed backhand winner down the line. An iffy backhand into the tramlines. Wawrinka up and then down, or down and then up, in this game. Cilic forces the game to deuce, before Wawrinka sees off the challenge with a sweet serve and volley.
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Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
"Kei Nishikori is someone who can get close to the top players, and it will be interesting to see how Grigor Dimitrov develops - he's a lot like a young Roger Federer, he has a good coaching set-up and has a lot of hunger."
Who can you see breaking into the top level of world tennis? Text and tweet us with your thoughts.
Wawrinka breaks
*Wawrinka 3-1 Cilic
Gasps from the crowd as Wawrinka returns a Cilic serve with the speed of a Cobra's tongue. Brilliant from the Australian Open champion. He fetches and retrieves from the baseline, defending stoutly, before Cilic falters - presenting his opponent with three break points. The Croat saves one, but approaches the net and volleys into the tramlines on the next point. A break of serve, a third game in the bag, and Wawrinka is through to the semi-finals.
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Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
"Wawrinka leapt onto that forehand and clubbed it down the line. Great tennis."
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Wawrinka 2-1 Cilic*
Solid serving once again from Wawrinka. A serve and volley eases him to 40-0. Cilic absorbs the punches on the next point, though, counter-attacking with another thunderous forehand down the line and Wawrinka's response is to overcook a forehand. Again, Cilic nicks just the one point on the Wawrinka serve, the Swiss ending the match with a picture-perfect forehand winner which grazes the tramline.
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Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
"This is a terrific tournament, it's more like the Davis Cup and much better for the spectators."
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*Wawrinka 1-1 Cilic
Both players unleashing the big shots early. Cilic with a forehand down the line, Wawrinka challenges the call, but replays show the Croat kissed the line by 0.000001mm. Close. To 40-15 we go and it's a routine hold for the US Open champion. This could actually be a contest.
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David Law
BBC Radio 5 live sports extra commentator
"If nothing else tonight, this crowd deserves to see the best of Marin Cilic - it deserves to see the US Open champion playing like the US Open champion."
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Wawrinka 1-0 Cilic*
Wawrinka sprints towards the back of the court to start this match. Eager. The Swiss made a fabulous start to this tournament, securing a sensational 6-1 6-1 victory over Berdych. Then he came up against Novak Djokovic and lost 6-3 6-0, but everyone seems to lose to the world number one these days so it should have no bearing on the outcome of this contest.
Two easy points for the Australian Open champion before he notches a third with a fabulous forehand winner into the corner. Wawrinka's single-handed backhand falters, gifting Cilic his first point of the game, but there's no danger of the Croat overhauling Wawrinka from here. Game Wawrinka. Two more and the Swiss is through to the last four.
Bye Bye Tomas
There'll be no more of Tomas Berdych at the 02 this year as two defeats in Group A means the Czech it outta here. The sixth seed has tweeted: "I had great year 2014. Thanx to my team, fans for your fantastic support in good and bad times!!! With ❤️ Tomas"
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Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra
"Like most people who know Andy Murray well, I felt for him last night. But it's been a terrific end of year for Andy, to win that number of matches in the last five or six weeks shows you how far he's come. It's taken its toll on him, but it sets him up to have a good think in pre-season on what he needs to improve on. He does it most years - he goes away, comes back a better player, and he can look forward to 2015 now."
Spot the ex-Wimbledon champion
To photobomb or not photobomb? It's a modern-day dilemma. I guess if you're a former Wimbledon champion you can get away with it. Marin Cilic's coach Goran Ivanisevic gets in on the act while the Tour Finals judges try to take a group photo.
Cilic the social climber
It's been an incredible year for US Open champion Cilic but he hasn't been at his best in north Greenwich. The Croat has lost his two group matches, suffering a 6-3 6-1 defeat by Tomas Berdych and a 6-1 6-1 loss to Novak Djokovic. Incredibly, he has had his serve broken 10 times during this tournament. Will Wawrinka make him suffer some more before the season ends?
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Wawrinka and Cilic are at the net for the coin toss, Wawrinka continuing with those lunges. Never has a man's muscles been so prepared for battle. Anyway... it's Wawrinka who will serve first after the US Open champion decides to receive.
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Russell Fuller
BBC tennis correspondent on Radio 5 live sports extra
"Tomorrow evening's semi-final will be Roger Federer against, probably, Stan Wawrinka. There have been so many one-sided matches this week, that we should reserve judgement at least for the first 10 minutes - as Wawrinka could go through if he starts well."
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Last-minute stretching for Wawrinka, who is lunging in the corridor before the players are called to make their way to the court.
Cilic leads the pair out and, as he approaches the arena, he can probably hear the crowd clap along to the sound of the beating heart which is booming out of the sound system.
Those permutations
Roger Federer awaits in the second semi-final. But, in an attempt to clear a few things up, here is who needs to do what:
All Stan needs to do is to take this match to a third set. In fact, the percentages mean that if Cilic wins in two sets but Wawrinka wins three or more games along the way, Wawrinka goes through. Confused? No? Good.
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Wawrinka is within touching distance of the semi-finals. He's so so close. The Australian Open champion merely needs to win three games against Cilic to set up an all-Swiss semi-final with Roger Federer, although had Wawrinka watched his friend's 6-0 6-1 thrashing of Andy Murray last night he may prefer to enjoy London's Christmas markets on Saturday rather than play a compatriot who is in eye-wateringly good form.
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It's time to start the final countdown for the final showdown. After 11 matches, 23 sets and 187 games, the final group match is upon us.
Are we too late for fireworks or too early? Stanislas Wawrinka versus Marin Cilic isn't ordinarily a match which would light up London's skies, but in a few hours the semi-final line-up for the ATP World Tour Finals will be decided and that is reason enough for pyrotechnics if you ask me. And it's Friday.