Clay won the 2008 Olympic decathlon title by over 200 points
Olympic decathlon champion Bryan Clay will miss August's World Championships after pulling out of the United States trials with a hamstring injury.
Clay, who won the world title in 2005 but pulled out of his defence in 2007 through injury, said he heard two pops in his leg during training on Tuesday.
He said: "It's a tough decision. But if somebody's going to beat me, I want it to be when I'm having my best day."
"I think if I had a few more days I could have made it happen."
Clay felt the pops during long jump training and at first thought it was not too serious an injury, but when he resumed running, he realised its severity.
He has undergone anti-inflammatory medication, massage, accupuncture and time in a hyperbaric chamber in an effort to get himself fit.
However, he admitted defeat on Thursday, shortly before the start of the US trials which will see the top three finishers heading to Berlin.
Clay, who also won a silver at the 2004 Athens Olympics, hopes to defend his Olympic title at the London 2012 Games and in doing so, become the first American to win decathlon medals at three different Games.