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.