AP National Writer
LONDON (AP) - When he's eligible _ and healthy enough _ to run, it's very hard to catch LaShawn Merritt.
Those moments, however, have come around sporadically since the American won the 400-meter gold medal in Beijing. Merritt's latest problem, a tender left hamstring, caught up to him Saturday on the biggest stage _ the London Olympics.
Looking to flip the script on a disheartening four-year period that included an embarrassing, 21-month drug suspension, Merritt pulled up halfway through his 400-meter heat and will not be around to defend his title.
"It's very disappointing to ... be dealing with an issue and not be able to finish the race," Merritt said. "I'll regroup."
By now, regrouping is routine for one of track's fastest runners.
He failed three successive drug tests between October 2009 and January 2010 for a banned substance found in a male-enhancement product. The substance has been used as a masking agent, but Merritt convinced the those judging the case that he really did buy it at a convenience store for its intended purpose. His ban was reduced in time for him to compete at world championships last year, then he won another case challenging an International Olympic Committee rule that would have kept him out of the London Games.
At worlds last year, however, he finished second behind Kirani James of Grenada because he hadn't been back long enough to get into good competition shape.
Now this. His first loss in 2012.
It was a result of an injury he suffered last month during a tuneup race in Monaco. He has spent the time between then and now in rehabilitation, but earlier this week, coach Loren Seagrave conceded he wasn't sure how Merritt would do once the lights went on.
Not well, as it turned out.
At the 150-meter mark, Merritt started slowing down and by the time he reached the far turn, he was done, hands on his hips, for a slow walk out of the stadium.
"I thought I could get through these rounds, not at 100 percent," Merritt said. "I got out and got around the curve and started to feel it. I moved a little more and still felt it. I think I need more rest."
With Merritt's exit, the 400 meters turns into a free-for-all, with James now the favorite and a bunch of interesting possibilities beyond that. Jonathan Borlee of Belgium set a national record in qualifying at 44.43 seconds. His brother, Kevin, also had a top-six time.
"Yeah, it may be a little bit more fun to watch," said Erison Hurtault of Dominica. "Now we're going to see, I guess it's someone else's turn now."
Could the same be true in the 4x400-meter relay, where America is trying for gold in an eighth-straight Olympics? Merritt's status for that race, which starts Thursday, is cloudy, and that places the team's hopes on less-sure footing.
"If I go out and really hit it in practice and still feel it, I'll let somebody else run," he said.
Either way, he sounded like a man who has more races in him.
"It's not the end of the world. The Olympics is going to come around again. Next year is a world championship," he said. "We have a season every year. So now it's a matter of getting healthy and getting back to what I love to do."
AP Sports Writers Pat Graham and Mark Long contributed to this report.
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