Gay will be fighting fit - coach

World champion Tyson Gay will be fully fit for the biggest challenge of his life when he runs in the highly anticipated 100 metres at the Beijing Olympics, his co-coach told Reuters.

"By the time he steps on the line, he will be 100 per cent," twice Olympian Jon Drummond said. "Otherwise, why show up?"

Gay strained his hamstring in the preliminaries of the 200m at the US Olympics trials in early July.

The 100 metres is expected to be the athletics highlight of the Games with Jamaican world record holder Usain Bolt, Asafa Powell and Gay all gunning for the gold medal.

The 100m heats begin today week with the final the following day.

Gay set an American record of 9.77 seconds in the quarter-finals of the US Olympic trials. He then won the final in a wind-assisted 9.68 seconds, the fastest time ever under any conditions.