Two-time defending champ Mardy Fish surrendered the Atlanta Open tennis crown when an injured left ankle forced him to retire from his second round match on Thursday.
The second-seeded American was leading Luxembourg's Gilles Muller 6-4, 3-2 when he called it quits at the hardcourt event.
Fish, 30, stumbled while trying to return a drop shot and rolled his left ankle at the net, dropping to the ground and staying down for about a minute.
He got up and limped off to his sideline chair, sat down for a short break and returned to finish the fifth game but then chose to withdraw.
Fish, who beat American compatriot John Isner in the last two Atlanta finals, recently underwent a procedure called cardiac catheter ablation, which deals with misfiring electrical pulses in the heart.
Isner, meanwhile, advanced Thursday to the quarter-finals by defeating Ruben Bemelmans of Belgium 4-6, 6-3, 6-4.
Aussie Matthew Ebden beat 32-year-old American James Blake 6-7 (6/8), 6-4, 6-4, and will play Muller on Friday.
American Jack Sock also won his second round match by beating Steve Johnson 7-6 (7/5), 7-6 (7/5).