Defending champion LaShawn Merritt of the U.S. pulled up in the backstretch of his 400-meter preliminary heat, stopped in his quest for a second straight Olympic title by an ailing left hamstring.
Merritt got off to a bad start, fell behind early and stopped running after about 200 meters. He limped slowly off the track, an unsatisfying end to a tumultuous four years for the sprinter.
Merritt served a 21-month drug suspension between Olympics and needed a ruling from the Court of Arbitration for Sport to become eligible for these games.
He was the gold-medal favorite coming into London, but hurt himself in a tuneup race. His coach said this week that Merritt's status for the Olympics was shaky.
Merritt was expected to lead the Americans in the 4x400-meter relay, which starts Thursday.