Little-known Scot Robison led a loaded field of Olympic medalists, including Ryan Lochte, with the fastest time in preliminaries of the 100-meter freestyle at the U.S. Olympic swimming trials on Thursday.

The 23-year-old sprinter from Charlotte, N.C., won his heat in 49.08 seconds, beating out 2008 Olympian Nathan Adrian, who was second-quickest at 49.17.

Robison was on the U.S. team at last year's world championships in Shanghai, and earned a bronze medal as part of the 400 free relay after swimming in the morning prelims. He competed in the World University Games in 2009, winning two gold medals as part of relays, but that is the extent of his international experience.

Still, Robison wasn't intimidated by the presence of veteran Olympians like Lochte, Jason Lezak and Cullen Jones.

"If right now was the first time I had thought about it, I'd be in trouble," he said. "But luckily, the past three or four years, I've been training knowing that all along. You just have to believe that you belong in that group."

Michael Phelps had the morning off, but he will return for the evening final of the 200 butterfly. As the two-time defending Olympic champion and world record holder, he is the heavy favorite for the race in which Lochte won't be competing.

Matt Grevers, the 100 backstroke winner at trials, was third in the 100 free at 49.24 and hoping to land a spot in the relay pool for London.

"I don't think I have the speed in the freestyle right now to compete for top two," he said. "But a relay spot would be great. I'm pretty tall. I have a pretty good relay start. So I think I could be a valuable relay swimmer."

Jimmy Feigen was fourth at 49.29 and Lochte fifth in 49.33.

2008 Olympian Garrett Weber-Gale was sixth at 49.34, and two-time Olympian Ricky Berens was seventh at 49.35. Lezak, the 36-year-old sprinter who chased down Frenchman Alain Bernard on the final leg of the 400 free relay to win gold in Beijing, was ninth at 49.40.

"I've got to be faster tonight, and I will be," said Lezak, one of 16 men who advanced to the evening semifinals.

Jones, trying to regain his form at 28 after not being a major presence in the sprints, said he tightened up toward the end of the race. He was 10th at 49.41. Only the top 8 from the semis move on to Friday night's final.

"Need a little less coffee and a little more Gatorade," he said. "I feel older. No, I feel good. Done the practicing. Done everything I need to do. Just little small things: not go out so fast and be able to bring it home faster."

Anthony Ervin, the 2000 Olympic 50 free champion, was 11th at 49.54. He is making a comeback at 31 after being retired since 2003.

"How do you spell suffer?" he said, laughing. "Just the tension, just the entire lead-up to that first race. Everything is kind of going crazy around me, and then everything kind of gets quiet. Then you're just lost in the moment as you're behind those blocks.

"I got a little taste of it there. Hopefully, I will have a little bit more for tonight, do a little bit better if I'm going to make it into finals."

Ervin is also entered in the 50 free on Saturday.

"I train kind of both just because they've always been kind of connected for me," he said. "If I do well in one, I do well in the other."

A couple of lesser names were the two fastest qualifiers in the 200 breaststroke.

Former NCAA champion Clark Burckle led the way with a time of 2:10.30. The brother of 2008 Olympian Caroline Burckle was fifth in the event at trials four years ago and has limited international experience.

Former UC Davis swimmer Scott Weltz was second-quickest at 2:10.90. Eric Shanteau, who was 10th in Beijing, advanced in third at 2:11.57. He already made the team in the 100 breast with Brendan Hansen.

"Yeah, the pressure is off a little bit, but you don't want to lose that edge," Shanteau said. "You still need to keep that, and I think I'm doing a good job with that."

Hansen qualified fifth at 2:12.14.

Ed Moses, the 2000 Olympic silver medalist in the 100 breast, just missed making the evening semifinals by two spots. The 32-year-old swimmer led the first 50 meters of his heat, but wound up 18th overall in 2:14.32.

"When I did the trials last (in 2004), we were in Long Beach, maybe 5,000 or 6,000 people," he recalled. "I get to swim in this. It was a good way to end my career for sure. Swimming is real big now. To say I was a part of it is going to be pretty cool. It was absolutely worth it."

Kim Vandenberg, who at 28 was the oldest woman by five years to make the semifinals, had the fastest prelim time in the 200 butterfly at 2 minutes, 8.78 seconds. Vandenberg, who made the 2008 Olympics as a relay team member, finished third in the fly four years ago at trials, just missing qualifying for an individual event.

Texas A&M sophomore Cammile Adams was second at 2:08.84, followed by Tennessee junior Kelsey Floyd at 2:09.02.

Teresa Crippen, the younger sister of late open-water swimmer Fran Crippen, qualified fourth at 2:09.48. Kathleen Hersey, who finished last in the Olympic final in Beijing, also advanced to the evening semifinals in fifth at 2:20.25.

Elaine Breeden, seventh in the final four years ago in Beijing, was 17th and missed the semifinals by one spot.