Tyson Gay gritted his teeth to run through the pain barrier as he warmed up for the defence of his world 200 metres title with a 20.00 seconds victory over the distance at the London Grand Prix on Saturday.

In an unwelcome echo of the 2008 Olympics a persistent groin problem is clouding the American's preparations for the world championships in Berlin where his double sprint showdown with Olympic champion and world record holder Usain Bolt was expected to be among the highlights of the Aug. 15-23 event.

Gay is the reigning world champion over 100 and 200 metres and the fastest man over both distances this season.

His 19.58 two months ago was the third-fastest 200m ever behind Michael Johnson (19.32) and Bolt's world record 19.30 while his 9.77 is the fastest 100m of the season.

On Saturday he flew out of the blocks and produced a blistering first 100 that allowed him to ease home well clear of compatriot Wallace Spearmon (20.35) with Ireland's Paul Hession third in 20.40.

"I didn't warm up like I wanted...the way I slowed down it would have been 19.80 at least," Gay said.

"I haven't trained for a couple of weeks. My groin is a bit tight on me. I'm just trying to run through the pain and be tough about it.

"I'm just taking it one day at a time. Any time there is any sort of pain you get frustrated."

Gay will be desperate to arrive in Berlin fully fit after a hamstring problem ruined his chances in Beijing where he failed to make the final of the 100m having missed out on qualifying for the 200 after suffering the injury in the U.S. trials.

Bolt, who eased to victory in the 100m in 9.91 on Friday, is scheduled to run in the 4x100m relay later on Saturday.