NEW YORK (REUTERS) - Fourth seed Alexander Zverev advanced to the fourth round of the US Open on Saturday (Sept 2), when Jack Sock retired with a thigh injury trailing 3-6, 6-2, 6-3, 2-1.

Zverev had looked to be headed for a long night against the big-hitting Sock, who convincingly won the opening set, but instead outlasted the American who refused to wave the white flag until broken in the fourth knowing there was not way back.