Two-time defending champion Mardy Fish withdrew from the Atlanta Open on Thursday due to an apparent left ankle injury.
Fish, the No. 2 seed, was leading Gilles Muller of Luxembourg 6-4, 3-2 in a quarterfinal when he pulled out of the match.
Fish stumbled to his left while trying to return a drop and his left ankle appeared to buckle as he got near the net. Fish dropped his racquet and fell to the hard-court surface.
He stayed on his back for about a minute near the left post before limping to the sideline for a short break. Fish returned to finish the fifth game of the second set with two aces. After he and Muller changed sides, Fish waved his opponent off.
Fish, the world's No. 13-ranked player, underwent a heart procedure on May 23 to correct an arrhythmia.
Muller will play Matthew Ebden, a 6-7 (6), 6-4, 6-4 winner over James Blake.