John Isner won a hard-fought all-American contest against Jack Sock 7-6 (6), 4-6, 6-4 to reach the semifinals of the Paris Masters on Friday.
Isner looked set for a straight-set win while served up a break at 4-3 in the second set but lost his temper after a foot-fault call.
Isner, who had not dropped his serve once since the start of the week, was unsettled by the decision and was broken twice in a row, allowing Sock back in the match.
But the change in momentum did not last long. Isner capitalized on Sock's mistakes to break him twice early in the decider.
Isner equaled his best result at the indoor tournament, making it to the last four for the first time in five years in the French capital.