American Michael Phelps, competing in a men's 100-metre butterfly on Thursday, is in the hunt for more gold. (Itsuo Inouye/Associated Press)

For a change, American star swimmer Michael Phelps didn't finish first, but easily advanced out of Olympic men's 100-metre butterfly Thursday.

With five gold medals in Beijing, Phelps remains in the hunt for No. 6 at the National Aquatics Center.

He qualified second overall Thursday in 50.87 seconds, trailing Serbia's Milorad Cavic, who won their heat in an Olympic-record 50.76.

Two heats earlier, Jason Dunford of Kenya briefly owned the Olympic mark in 51.14. That erased Phelps's record of 51.25 set four years ago in Athens, when he won the event.

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.

Canadians Joe Bartoch and Adam Sioui failed to crack the top 16 in order to advance, finishing 34th and 39th respectively.

Afterwards, Sioui remained non-committal about whether he'll remain in the pool at age 30 and attempt a better finish at the 2012 Summer Games in London.

"It's been a long time coming for me and it's been pretty gratifying to have made it this far," he told CBC Sports' Elliotte Friedman of his first Olympic experience in China. "I'm not going to make any rash decisions. I'm going to enjoy the rest of the [Olympic] experience that a lot of people in the world [don't get a chance] to do.

"[And] I'm going to sit down with family and friends and talk about it."

Bartoch was more forthcoming about his status for the next Olympics.

"I was going to save this but I really want the world record," Bartoch said. "I don't know anybody else that'll say that will say that publicly, but I really want to go for the world record."

He described his first Olympic experience in Beijing as a disappointment.

"That's the best I had for today, so we'll move on," he said.

Tanya Hunks turned in the best result for Canada in the evening session, placing 23rd in the women's 800-metre freestyle in a time of eight minutes 38.05 seconds.

Britain's Rebecca Adlington touched the wall first in 8:18.06, an Olympic record. Brooke Bennett held the previous mark of 8:19.67, set at the 2000 Sydney Games.

Americans fail to advance

Of note was the performance of Americans Kate Ziegler (8:26.98) and Katie Hoff (8:27.78), who weren't among the eight swimmers advancing, having finished 10th and 11th, respectively.

It was a stunning result for the United States, which has won the event seven times since it was first contested at the 1968 games, including five straight from 1984-2000.

Hoff finished third in her heat, touching in eight minutes 27.78 seconds, a whopping 8.08 seconds off her personal best time done in April.

One heat later, Ziegler also went out fast, but began slowing at the 450m 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 current 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."

Meanwhile, Canadian Richard Hortness stood 27th in the field of 97 in the men's 50 free after clocking 22.42 seconds, well behind Frenchman Leveaux Amaury's winning time of 21.46, also a world record.

In the women's 200 backstroke, Lindsay Seeman of Canada was 30th in 2:15.07.

With files from the Associated Press