BEIJING — Katie Hoff and Kate Ziegler ended their Olympics with a stunner — neither teenager advanced out of the 800-meter freestyle heats Thursday night, ensuring the United States a medals shutout in the grueling event for the first time in 28 years.

Hoff was 11th overall and Ziegler 10th, with both finishing more than 8 seconds off their personal best times.

"I don't think many people bet on that," Ziegler said.

No way.

The U.S. has medaled in every Olympics except the 1980 boycott year and 1976, and won golds in five straight from 1984-2000.

Hoff ended her second games without a coveted gold medal, despite swimming five individual events. She settled for a silver, two bronzes and a pair of fourth-place finishes in her other races.

Hoff declined to speak with reporters after the race.

She finished third in her heat, touching in 8 minutes, 27.78 seconds — a whopping 8.08 seconds off her personal best time done in April. She took the 16-lap race out strongly, but started slipping at 500 meters.

At the finish, Hoff gasped for air, her chest heaving as she clung to the starting block. She was the last to leave the pool, with an official urging her to move along. Finally, she did, but paused to rest on another starting block before making her way to the edge and climbing out slowly.

One heat later, Ziegler was fourth in 8:26.98. She also went out fast, but began slowing at the 450 mark. Only the top eight advanced to the final. She was 8.46 seconds off her personal best done in March 2007.

"The first 400 felt great. I was like, 'OK, this is going to be a good swim,'" said Ziegler, the 800 free world champion. "It just didn't happen. Some days your body just doesn't work. My body picked a bad time not to work."

Ziegler wasn't quite sure what went wrong, but she said she might have spent too much time resting at the team's pre-Olympic training camp in Singapore.

"I do really well just kind of being in a stable environment," she said. "Not traveling much, not switching things up much. Maybe if it hadn't been quite as long it would have been better."

Ziegler also didn't get out of the 400 free prelims, winding up 14th. Hoff finished second.

"Even though I had disappointing swims, I went to the Olympics," Ziegler said. "I feel like I'm just beginning on my way. Hopefully, I'll go on to bigger and better things in the future."

Michael Phelps certainly is.

He advanced out of the 100-meter butterfly preliminaries, qualifying second overall in 50.87 seconds to keep him on track to win his sixth gold medal of the games.

Phelps trailed Milorad Cavic of Serbia, who won their heat in an Olympic record of 50.76.

Two heats earlier, Jason Dunford of Kenya briefly owned the Olympic mark in 51.14. That erased Phelps' record of 51.25 set four years ago in Athens, when he won the event. And world record-holder Ian Crocker of the United States moved on in 13th place, timed in 51.95. The top 16 advanced to Friday morning's semifinals.

Amaury Leveaux of France qualified fastest with an Olympic-record time in the 50 free preliminaries, ahead of 100 champion Alain Bernard and world record-holder Eamon Sullivan.

Leveaux cruised through swimming's most chaotic race in 21.46 seconds, bettering the Olympic mark set one heat earlier by Cesar Cielo of Brazil.

Cielo, who tied American Jason Lezak for bronze in the 100 earlier Thursday, was second-quickest in 21.47. The top eight all went under Russian Alexander Popov's mark of 21.91 set at the 1992 Olympics, with Leveaux eventually claiming the record.

American Ben Wildman-Tobriner, the current world champion, moved on to Friday morning's semifinals in third at 21.75, tied with Stefan Nystrand of Sweden.

"It was definitely a preliminary swim," Wildman-Tobriner said. "I'm going to take a look at the video. I know there's a couple things I can work on. Throughout semis and finals, everyone is going to keep going faster."

Roland Schoeman of South Africa, the 28-year-old bronze medalist four years ago in Athens, was fifth.

"You look at Bernard and Sullivan and they've been performing at their best for six months now, so they're on the top," he said. "It's up to us older folks to come in there and slip it away from them."

Bernard returned hours after winning the 100 to qualify seventh in 21.78, giving the Frenchman a chance to become the first to pull off the 50-100 double since Popov did in 1996 and '92.

Aussie Sullivan, who took silver in the 100, was right behind Bernard at 21.78.

"It's something I've never experienced, swimming two different races in the same day, especially coming off such a big final," Sullivan said. "But I think I coped well, got through the heats well and got a lot more in store for tomorrow."

Also advancing were American Garrett Weber-Gale (ninth) and 2004 silver medalist Duje Draganja of Croatia (12th). Defending champion Gary Hall Jr. didn't qualify at the U.S. trials.

"Those guys are tough. They're good swimmers," Weber-Gale said. "But if you don't believe you're going to swim fast and you can do it, just don't come out and swim because you're not going to give yourself a chance."

Popov's world record stood unchallenged for nine years until 10 swimmers bettered it this year in an event where 22 seconds used to be fast enough for a medal. Ten of the 16 qualifiers swam under that mark in a race where most of them gulp just one breath.

"You can't hold back in the semis, especially the 50," Sullivan said. "It's probably going to be 0.1 between the top eight. It's going to be ridiculously fast."