The sprinter, 28, who would have been one of Usain Bolt's main rivals in South Korea, withdrew before his 100m semi-final at the US national trials.

　　Gay had an arthroscopic procedure on his hip and will now focus on preparing for next summer's Olympics in London.

　　Earlier this season he clocked 9.79 seconds for the 100m - the fastest time in the world this year.

　　His personal best - 9.69 seconds - is a US national record and the second fastest time ever recorded, behind Bolt's world record of 9.58 seconds.

　　Gay had qualified from his heat at the US trials in a wind-assisted 10.01 seconds, second behind Ivory Williams and third fastest overall, but pulled out before the semi-final, citing a recurrence of a problem in his right hip and adductor muscle.

　　TYSON GAY FACTFILE

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　　2007 World Championships 100m, 200m and 4x100m gold medallist

　　2009 world 100m silver medallist

　　Second fastest 100m runner in history (9.69 seconds)

　　Ran the fastest 100m of 2010 (9.78 seconds)

　　Beat double Olympic and world champion Usain Bolt the last time they raced

　　"I've been bandaging it up (and) couldn't really take (it) anymore," Gay said. "I decided to stay healthy. It's pretty painful running in pain."

　　His semi-final was won by Williams, who was then disqualified from the final as Walter Dix won in 9.94 seconds.

　　Justin Gatlin, the disgraced 2004 Olympic champion who has returned to the sport after completing a drugs ban, finished just one hundredth of a second behind Dix in second with Michael Rodgers third to complete the US trio for Daegu.

　　Gay could still be part of the 4x100m relay team were he to recover in time, USA Track and Field (USATF) officials said.

　　"But he has to be able to show fitness and be able to run," said USATF President Stephanie Hightower.

　　Gay's doctor, Marc Philippon, appeared to rule out that possibility, saying Gay should only be able to resume training within the next six to nine weeks.

　　"It is a big blow," said Dix of Gay's potential loss to the relay squad. "Tyson at his best is one of the top runners in the world. We're going to have to step up."

　　Gay has been badly affected by injuries since winning the 100m and 200m world titles in Osaka in 2007.

　　In 2008, he was injured during the 200m at the US Olympic trials and later that year failed to make the 100m final at the Beijing Games.

　　The following year, he pulled out of the 200m after taking 100m silver behind Bolt when the Jamaican broke the world record at the World Championships in Berlin.

　　Former world record holder Asafa Powell edged out Bolt's training partner Yohan Blake to win the Jamaican 100m trials for the World Championships.

　　Powell clocked a winning time of 10.08 seconds into a headwind and was joined by Blake (10.09) and Steve Mullings (10.10) in claiming berths for Daegu.

　　World record holder Bolt did not compete in the trials because he has a wildcard bye as the defending champion in both the 100m and 200m.(本文由在线英语听力室整理编辑)