By Gene Cherry

EUGENE, Oregon (Reuters) - LaShawn Merritt ran the fastest 200 meters in the world this year and Brianna Rollins shocked American record holder Keni Harrison in a world class 100m hurdles final at the U.S. Olympic trials on Friday.

Merritt, already on the U.S. team as the trials' 400m winner, sped through his semi-final in 19.74 seconds to set the stage for a Saturday showdown with world silver medalist Justin Gatlin, up-and-coming Ameer Webb and two teenagers.

A celebrating Rollins dominated the hurdles showdown, winning in 12.34 seconds as a stunned Harrison finished sixth to miss a trip to Rio.

Olympic champions Allyson Felix and Aries Merritt, meanwhile, moved safely through qualifying in the 200 meters and 110 hurdles respectively on the often rainy day.

LaShawn Merritt would not say whether he, like Felix, would try for the 200-400m double in Rio.

But he appeared ready if he makes the team in the 200.

"I got some rest and went to work," said the 2008 400m Olympic gold medalist who holds the fastest times in the world this year in the 200 and 400.

Webb also dipped under 20 seconds (19.97 seconds) with former world champion Tyson Gay (20.16) and teenager Michael Norman (20.21) ahead of Gatlin (20.23). Another teen, Noah Lyles (20.26), trailed Gatlin.

"I'll rise to the occasion, when it is time," said Gatlin, who praised the semi-final winning teenagers.

"They are good. They could run with us on the Diamond League," said the 32-year-old. "They came out to be giant killers."

Of Merritt, Gatlin said: "He's been in 19 (second) shape since May. He will be ready."

Rollins certainly was.

OVERWHELMED

The former U.S. record holder showed she was not about to yield the 100m hurdles mantle to Harrison, who until the race appeared the odds-on favorite for Rio.

"When I got to the finish line and realized I was the first one to cross, I was so overwhelmed," said Rollins.

"We're the best in the world ... and hopefully all three of us end up on the medal stand.”

Kristi Castlin (12.50 seconds) and Nia Ali (12.55) joined her on the U.S. team.

Harrison, who narrowly missed the world record in May, was never a factor in 12.62.

"I don't know what happened," she said. "It's really unfortunate. My season was going so great."

Former Olympic champion Dawn Harper Nelson will also miss Rio. The 2008 gold medalist went out in the semi-finals.

The 32-year-old had represented the United States in six consecutive global championships.

OFF STRIDE

Felix, the 400m trial winner, ignored a sore right ankle to easily win her 200m preliminary, her first since September.

"It felt good. It felt comfortable," America's most decorated female sprinter said of her ankle which she hurt in April.

She clocked 22.93 seconds, fifth fastest of the day.

Local favorite Jenna Prandiini (22.73 seconds) and Tori Bowie (22.74) were among those finishing ahead of Felix.

World record holder Aries Merritt appeared a bit off stride, hitting four hurdles in qualifying sixth-fastest in the 110m hurdles with a time of 13.64 seconds.

"There was a lot of wind today, so I was making a lot of mistakes. But it was a solid round," said Merritt, who underwent a kidney transplant last September.

"I have another round to clean it up, then hopefully in the final I can do something magical," added Merritt, whose 2012 world record is 12.80 seconds.

Former world champions Jason Richardson (13.41) and David Oliver (13.68) also advanced with Ronnie Ash running the top time of 13.39 seconds.

(Editing by Ian Ransom)