Justin Gatlin and LaShawn Merritt remained on course for a 200 metres final while top hurdlers Brianna Rollins and Keni Harrison put on an early speed show at the US Olympic trials on Thursday.

World silver medallist Gatlin eased through qualifying, finishing only eighth as Merritt, Walter Dix and Tyson Gay also made it through the 200 metres opening round.

Rollins beat Harrison by one hundredth of a second, clocking 12.56 seconds in women's 100 metres qualifying.

Gatlin, already on the US team as the 100 metres trials winner, clocked 20.32 seconds to finish behind Ameer Webb (20.27 seconds) in their race on a cool, rainy afternoon.

"I just took my time and made sure I was in the top three coming out of the curve," Gatlin said. "I knew Ameer was gonna run. He's got one gear, so he's gonna go."

Four hundred metre trials winner Merritt, who holds the year's fastest 200m and 400m times, was third quickest in a wind-assisted 20.09 seconds.

"The 200 is still a little foreign to me, but I know that I'm fast and strong, so I figured I can handle it," Merritt said.

Dix (20.23) and Gay (20.36) moved on in wind-assisted performances in the July 1-10 trials which select the American team for the Rio Olympics.

Noah Lyles, just 18, had the day's fastest time, 20.04 seconds, but was pushed along by an assisting wind.

The 100 metres hurdles qualifying featured the year's five fastest hurdlers.

Harrison put down the gauntlet with a 12.57-second clocking and held the lead until Rollins, in the last hurdles qualifying race, overtook her time.

No one was within a tenth of a second of the two.

"I just want to continue to relax, focus on myself and just remember this is another track meet," said the former US record holder Rollins.

Harrison, who bettered Rollins' record in May, said she was a bit stiff in her preliminary.

"I didn't get out as hard as I would like to," said Harrison, one of America's rising stars.

"If I have a good start I think I could break the world record," said the 23-year-old, who missed Bulgarian Yordanka Donkova's 1988 record of 12.21 seconds by three-hundredths of a second in May.

"But I am not worried about that. I just want to come across the line top three (to make the US team)."

Former Olympic champion Dawn Harper Nelson could only muster the ninth fastest qualifier, 12.85 seconds, demonstrating America's speed in the event.

World indoor champion Michelle Carter won the women's shot put (19.59 metres) and Emma Coburn won the 3,000 metres steeplechase (9:17.48).