LONDON — Jack Sock won for the first time at the ATP Finals on Tuesday, beating Wimbledon finalist Marin Cilic 5-7, 6-2, 7-6 (4).
Sock lost his debut match to Roger Federer, but he is now 1-1 in his round-robin group.
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.
Cilic saved break point in the first set and then broke his American opponent for the lead. Sock fought back to take the second set, and then fought back again to force a tiebreaker in the third after being down a break.
The pattern continued into the deciding tiebreaker with Cilic getting the first mini-break before Sock turned it around.