GAME, SET AND MATCH! - CILIC 7-5 2-6 6-7 SOCK. It's Jack Sock who comes through a tense tie-break decider, hitting back from 4-2 down to prevail 7-4 after two hours and 27 minutes. The American keeps his hopes alive ahead of a Thursday showdown with Alexander Zverev. Cilic will start to think about his flight home.
Marin Cilic - Jack Sock
ATP World Tour Finals - 14 November 2017
World Tour Finals – Follow the Tennis match between Marin Cilic and Jack Sock live with Eurosport. The match starts at 15:00 on 14 November 2017. Our live coverage lets you follow all the key moments as they happen.
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TB LATEST: It's as tight as it has been for much of the match. Both players want this BIGTIME - but it's 4-2 to Cilic at the change of ends.
CILIC 7-5 2-6 6-6 SOCK. Jack gets it done to 15 and we will head into a tie-break decider.
CILIC 7-5 2-6 6-5 SOCK. Another love hold. Pressure moves back to Sock.
CILIC 7-5 2-6 5-5 SOCK. That's solid from Sock. At 30-30 he finds a big serve to nudge in front and then outfoxes Cilic at the net to keep his hopes alive.
CILIC 7-5 2-6 5-4 SOCK. Marin thunders down a 12th ace to seal a perfect hold and crank up the heat on Sock, who must hold to keep the match alive.
CILIC 7-5 2-6 4-4 SOCK. It's down to the wire now. Whoever blinks can start packing their bags for home...
CILIC 7-5 2-6 4-3 SOCK. Cilic stops the mini rot to edge back in front with his own love hold. Still hard to guess who is going to come out on top. The games are certainly flashing by at a faster right now.
CILIC 7-5 2-6 3-3 SOCK. All square. The pendulum swings the way of Sock as he races to a love hold. The American has responded well, but what does Cilic have left up his sleeve?
BREAK! - CILIC 7-5 2-6 3-2 SOCK. Cilic is forced on to the back foot and has to produce a brutal crosscourt forehand to save a break point from deuce. Sock has another nibble and earns the break back when the Croatian No.5 seed fizzes wide down the line.
It's been an underlying theme in the season.
CILIC 7-5 2-6 3-1 SOCK. He needed a quick response - and he's got one. Sock gets himself on the board with a simple hold to 15. Can he rediscover his Set 2 form and turn the tide?
CILIC 7-5 2-6 3-0 SOCK. Errors are leaking from Sock and it's given Cilic a near perfect start to this decider. He consolidates with a hold to love. Beware the wounded animal - great reaction from the Croatian.
BREAK! - CILIC 7-5 2-6 2-0 SOCK. A yell of joy from Cilic as he outwits Sock with some clever tactics on the return to strike an early blow.
CILIC 7-5 2-6 1-0 SOCK. Cilic has re-focused and comes out with a real statement with another love hold to open Set 3.
SET! - CILIC 7-5 2-6 SOCK. All square. Sock confidently strolls his way through a superb love service game, executing the drop shot to good effect before confirming the decider with an ace. Who's your money on now?
CILIC 7-5 2-5 SOCK. Cilic makes this set's score look a tad more respectable with a simple love hold that poses the serve-out question to Sock, who was clearly already thinking about what comes next.
CILIC 7-5 1-5 SOCK. Jack backs up the double break with a rampant love hold.
Some news from elsewhere in the world of tennis.
BREAK! - CILIC 7-5 1-4 SOCK. It's another break and Sock is looking good to take this to a decider. Cilic hooks horribly wide off the forehand on break point and may well need to re-think his game-plan.
CILIC 7-5 1-3 SOCK. A lengthy game goes the way of Sock after some brave, attacking tennis proves greater than his lapses in focus. He unleashes that trademark bullet of a forehand up the left-hand wing to fend off a BP and finally maintains he advantage from a fifth deuce with a sublime dropshot.
CILIC 7-5 1-2 SOCK. A pair of aces get Cilic on the board after Sock had upped the ante to pose a question at 30-30.
CILIC 7-5 0-2 SOCK. This time Sock consolidates his early break, keeping his focus after miscuing at 40-30 to see it through and channel his earlier frustration in the right way.
BREAK! - CILIC 7-5 0-1 SOCK. Third time lucky for Sock. He halts the Cilic momentum to once again break the Croatian at the start of a set after Chila had threatened to wriggle free by saving two BPs.
Fist pump time for Chila.
SET! - CILIC 7-5 SOCK. That really is superb from Cilic. He plays aggressive and smart, producing a lovely combination of shots off both flanks to tee up triple set point. Sock saves two but miscues a forehand low into the net on the third. He gets a warning for lashing out at the ball in frustration, but credit to Cilic for playing some intelligent, percentage tennis.
CILIC 6-5 SOCK. Chila comes through a tricky game to guarantee himself the minimum of a tie break. A delicious backhand crosscourt lob earns Sock a break point from deuce, but Cilic gets lucky with a big net cord and cruises through the next two points to make the most of his fortune.
CILIC 5-5 SOCK. Good reaction from Sock after Cilic had started well by picking up a dropshot and guiding it down the line. The American crunches down ace number four and some thumping forehands to gain parity once more. A tie break looms nearer on the horizon...
CILIC 5-4 SOCK. The trusty big serve keeps Cilic's nose in front after his adventurous lob had landed just long to give Sock a look-in at 30-30. The American must now hold to stay in the set.
CILIC 4-4 SOCK. That's more like it from Sock point of view. He blitzes his way to his own love hold. All square as we enter the business end of the set.
The media in and around the O2 are still talking about Rafa Nadal's early withdrawal with his problematic knee last night.
CILIC 4-3 SOCK. Cilic has looked the more assured on serve since the flurry of early breaks. This one ends as a love hold as Sock jumps into another return but sizzles it way long of the baseline. It's an eye-catching shot, but only if it works.
CILIC 3-3 SOCK. Deja vu as Cilic develops a point brilliantly at 40-30 down, before letting Sock off the hook with a poor volley at the net.
CILIC 3-2 SOCK. Routine hold for Chila. He rattles down a second ace of the afternoon and a sizzling service winner to maintain his impressive showing since that first game blip.
CILIC 2-2 SOCK. A quite sublime lob-volley is the pick of the points as Sock moves level. Cilic has a chance to make deuce but angles a very make-able volley into the net with the American stranded near the baseline.
CILIC 2-1 SOCK. Cilic looks to have found some form off his forehand, firing a flurry of howitzers that Sock has no answer to. A first ace of the match seals a stylish hold to 15.
BREAK! - CILIC 1-1 SOCK. The Cilic backhand has already found the net too many times for his liking, but he springs from defence to attack with a sweet strike off that flank to threaten from deuce. The Croatian gets a lucky with a nick off the net on his way to earning a break back point, which he gets as Sock unleashes a forehand missile just long. A fine response from this year's Wimbledon finalist.
BREAK! - CILIC 0-1 SOCK. The confidence is oozing from Sock. The Paris Masters title followed by his impressive showing against Federer on Sunday sees him come out with real belief. He hits one of the hardest return winners you're likely to see as he churns out two break point opportunities. Cilic saves one to make deuce, but then slaps a tight crosscourt attempt into the net. Advantage Sock right from the off.
Here we go. Cilic is primed and ready on the service line.... And... play!
Cilic wins the toss and opts to serve first. Time for the pre-match formalities and the warm-up.
DID YOU KNOW? Sock is the first American to make these finals since Mardy Fish in 2011.
Cilic produced a solid display in his three-set loss to Alexander Zverev, who produced purple patches at the start and end of the match to edge to victory.
Sock gave a strong account of himself on his end-of-season finals debut when he narrowly lost to Roger Federer 6-4 7-6 on Sunday afternoon. Will we see the American's 'bum tactic' today?!
HEAD-TO-HEAD: Sock has emerged triumphant in both of their previous encounters. The American came from two sets down to win a thriller in the 2016 Davis Cup and he also prevailed in straight sets in the US Open that very same year.
Hello and welcome to LIVE updates of the first singles match of Day three of the Nitto ATP World Tour finals. Marin Cilic takes on Jack Sock in a match neither player can afford to lose if they are to realistically keep their semi final hopes alive. The players are due on court for this must-win Boris Becker Group showdown shortly.