World number one Jack Sock earned his first ever ATP Finals win after he came back from a set down to defeat Marin Cilic on Tuesday.
The American battled back after losing the first set to defeat Cilic 5-7, 6-2, 7-6(4) in two and a half hours on the hard courts of the O2 in London, bouncing back from a straight set loss to Roger Federer on Sunday.
The pair swapped breaks in their opening service games in the first set before Cilic would clinch the final game to take the set.
But the momentum switched quickly in the second set, with Sock racing to a 4-1 lead on his way to forcing a deciding set.
Cilic came out of the blocks firing in the final set, earning a 3-1 lead of some powerful baseline play. Sock rallied to break back and eventually forced a tiebreaker.
At 4-2 down, it appeared the Croatian was about to get his first win at the tournament but a brave Sock had luck on his side with a net cord that would tie it up at 4-4 before pouncing on some lazy shots from Cilic to snatch the victory on his first match point.
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The win affectively knocks world number five Cilic out of the event, while Sock will have to beat Alexander Zverev if he wants to advance to the semi-finals later this week.