Jack Sock became the first American to win a match at the Nitto ATP Finals in 10 years when he outlasted fifth-seeded Marin Cilic on Tuesday in pool play at London.
Sock fought off the Croatian in a third-set tiebreaker to earn a 5-7, 6-2, 7-6 (4) victory at the O2 Arena. He is 1-1 in Group Boris Becker play.
Andy Roddick in 2007 was the previous American to win a match in the ATP Finals.
The eighth-seeded Sock's next task is reaching the semifinals.
"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," Sock said after the two-hour, 28-minute match. "I think I've showed that more this year."
The 25-year-old Sock fell behind 4-3 in the tiebreak but produced four straight points to win the match and improve to 3-0 all-time against Cilic.
"I think Jack is playing really good," Cilic said. "He had a really nice season, especially the beginning, now the end. ... Overall I think he's playing good tennis, quite aggressive. When he's playing well, he's definitely very dangerous."
Sock will face third-seeded German Alexander Zverev on Thursday.
Zverev was slated to face second-seeded Roger Federer later Tuesday in pool play.