LaShawn Merritt, the American sprinter who was favourite to retain his 400m title at the London Olympics, pulled up in his heat on Saturday and limped off the track.
Merritt, who has struggled back from a doping ban disgrace, stopped running after 250 metres of his heat and crossed the line at walking pace.
"This was my life's race," the 26-year-old said. "I have been around a long time, but I am still young."
The American won the chance to compete at London by taking the title at last month's US Olympic trials in 44.12sec, the fastest time in the world this year.
But he pulled up in the Monaco Diamond League meet last month, similarly coming to a halt after 300 metres.
"I've been having treatment since I got here. When you want it so bad, you do what it takes," Merritt explained. "But I need some more rest. It's feeling a whole lot better but when I really went for it, I felt it.
"It's probably a (hamstring) strain. I felt it twitch during my warm-up. I tried to get through it, but I couldn't.
"It's just one of those things. Now, may the best man win," Merritt said, adding he would be "ready for the world championships (in Moscow in 2013) and certainly for the next Olympic Games".
Merritt hinted he would likely miss the 4x400m relay.
"If I go out and really hit it in practice, okay, but we have a lot of depth in the 400m, it wouldn't make sense on my part not to let those guys run," he said.
"It's not the end of the world. I'll go away and regroup."
Merritt tested positive for steroids three times in late 2009 and served a 21-month doping ban before winning an appeal to an arbitrator and successfully fighting an Olympic rule that would have banned him from the London Games.
That victory at the Court of Arbitration for Sport came at the cost of an embarrassing revelation, with Merritt saying he'd tested positive because he took a $6 male enhancement product, ExtenZe, that contained the banned substance DHEA.
Merritt came back last year from the ban to take second at the world championships in Daegu.
Other than Merritt, the remaining medal contenders went through, including Belgian Jonathan Borlee in a national record of 44.43sec, and twin brother Kevin.