World champion Tyson Gay equalled the fourth fastest 100 metres of all time when he clocked an American record 9.77 seconds in the quarter-finals at the US Olympic trials on Saturday.

The sizzling run followed an embarrassing mistake by Gay in the opening round in which he said he misjudged the finish line and needed a late surge to qualify fourth.

Only world record holder Usain Bolt and Jamaican countryman Asafa Powell, the former record holder, have run faster.

"I thank God that I was able to run a personal record, but that's what I want to do in the finals," said Gay, whose previous best was 9.84 secs.

"After the first round, I was scared," Gay said.

"I almost started crying as soon as I crossed the line because I thought I didn't make it."

His time broke former world record holder Maurice Greene's 1999 national record of 9.79 seconds and overshadowed Muna Lee's surprising win in the women's 100 metres final.

Lee clocked 10.85 seconds to upset former world champions Torri Edwards and Lauryn Williams.

Edwards finished second and Williams third to make the US team for Beijing. Both clocked 10.90 seconds.

Edwards had equalled the eighth fastest women's 100 ever with a time of 10.78 seconds in the semi-finals.

Marshevet Hooker, who had a wind-assisted 10.76 seconds on Friday, was a non-qualifying fourth and world 200 metres champion Allyson Felix took fifth.