World champion Tyson Gay ran a wind-assisted 9.75 seconds in what he called a "horrible" 100 metres preliminary at the U.S. world championships trials on Thursday.

Although the mark was the seventh fastest of all time under any conditions, Gay said he was disappointed with his execution in the race assisted by a wind of 3.4 metres per second.

"I ran a horrible race. Technically it was horrible. My focus was horrible," Gay told reporters.

But the fast time and a quick 200 metres last month should see Gay line up for a world championship showdown against Jamaican Olympic champion and world record holder Usain Bolt in both sprints in Berlin in August.

With a bye to Berlin in both the 100 and 200 as the defending world champion, Gay had planned Thursday's opening round of the 100 metres as his only appearance at the trials.

He talked of running another round, but after consulting with coaches, decided against it.

The assisting wind led to a number of fast first-round times. Olympian Darvis Patton and upcoming Mike Rodgers both clocked wind-assisted 9.92 seconds.

The world record is Bolt's 9.69 seconds at the Beijing Olympics.

Defending world 1,500 and 5,000 metres champion Bernard Lagat claimed his wild-card berth for both events in Berlin with an easy run of 1:48.59 in 800 metres qualifying. He does not plan to run again in the trials.

Olympic champion Stephanie Brown Trafton topped the women's discus final with a toss of 64.25 metres and Beijing gold medallist LaShawn Merritt clocked 45.23 seconds to lead qualifying in the men's 400 metres.

CLAY OUT

Earlier, the U.S. lost one of its top hopes for a world championship gold medal when Olympic decathlon gold medallist Bryan Clay withdrew from the trials with a hamstring injury.

Only the top three finishers at the national trials or defending world champions qualify for Berlin.

"It is a big disappointment," Clay, 29, told Reuters.

"I really would like to be on the team, but at the same time I am trying to keep my mind wrapped around the big picture. I do want to go after the world record."

Clay said he injured his left hamstring while working out on Tuesday.

"I know I am in shape. I know I am ready to compete. I just had a little hiccup that is going to take me out for 10 days or so... In six weeks, I would be fine going to the world championships."

The trials continue until Sunday.