Defending champion Bryan Clay injured his leg attempting a high jump on Friday and was forced out of the decathlon at the track and field world championships.
Clay's agent, Paul Doyle, said the American decathlete hurt a quadriceps muscles in high in his right leg and "heard something pop."
Clay was leading after the 100 and long jump disciplines but fell to second place in the shot put. He slipped back to third when he withdrew from the high jump on before his third attempt at 2.00 meters.
Doyle said the extent of the injury was unknown.
Jamaica's Maurice Smith led after the high jump with 3,591 points, a 28-point edge over Czech Olympic champion Roman Sebrle.
When he withdrew, Clay had 3,558 points.