LONDON (AP) The United States has been waiting 10 years for a win at the ATP Finals, few thought Jack Sock would be the man to deliver it.
The No. 8-seeded player defeated Wimbledon finalist Marin Cilic 5-7, 6-2, 7-6 (4) on Tuesday.
Sock lost his debut match at the O2 Arena to Roger Federer, but he is now 1-1 in his round-robin group.
While others might have been surprised at Sock becoming the first American to win at the elite season-ending event since Andy Roddick in 2007, the man himself wasn't.
''I wouldn't be out here playing, competing if I didn't think I could give myself a chance to win tournaments, to be here,'' Sock said.
''I'm not the type of guy that will just enter a draw and be happy to make the third round or the quarters or whatever. I'm here to win matches. I think if I do the right things and I play the right tennis, I can give myself a chance to play on the weekend of any tournament.''
Cilic, who has won only one of his eight matches overall at the season-ending tournament, is 0-2 and will be eliminated if Federer beats Alexander Zverev in the late match.
''Definitely not a great record, but I felt that I played even well last year,'' Cilic said. ''In those three matches that I played, I had good chances at least to win. These two also. So just, you know, couple points away from closing those matches. A little bit unfortunate with that.''
Despite having been forced out of bed in the early hours of Tuesday morning by a fire alarm at his hotel, Sock made a fast start - breaking in the opening game, only to hand it straight back to his opponent.
The next break point didn't come until the 11th game. A moment of huge fortune saved Cilic as his forehand got a dead net cord, which Sock couldn't track down.
The Croat made good use of his luck, breaking in the following game to claim the opening set. A visibly frustrated Sock received a code violation as he blasted a ball into the upper tier, but it seemed to work for him. The American broke Cilic in the opening game once more and did so again in fifth to level the match.
Cilic broke for a 2-0 lead in the deciding set, but Sock displayed the fighting spirit that carried him to an unlikely Masters victory in Paris last month, breaking back in the fifth game and force a tiebreak.
With Cilic a 4-3 and a minibreak up, Sock produced the shot of the match as he sprinted forward to retrieve another near-dead net cord and didn't look back from there, winning the next three points to secure the match.
''As for the tiebreak, just hope he doesn't hit the net again,'' Sock said. ''He had a few of those today, but that's tennis.''