RIO DE JANEIRO (AP) — Swimming got off to a fast start in preliminaries at the Olympic pool on Saturday, with a world record in the men's 100-meter breaststroke, the second-fastest performance ever in the women's 400 individual medley and a defending champion knocked out.

Adam Peaty of Britain lowered his own world mark with a time of 57.55 seconds in the 100 breast heats, leading 15 others into the semifinals later in the evening.

Katinka Hosszu of Hungary just missed setting a world record in the 400 IM. Her time of 4 minutes, 28.58 seconds was the second-quickest ever in the event and just 15-hundreths off of Ye Shiwen's mark of 4:28.43. Ye won't defend her Olympic title after she failed to qualify for the final. She finished 27th out of 33 swimmers.

Katie Ledecky opened her Olympic program by swimming the anchor leg of the 4x100 freestyle relay. She rallied the Americans from third to second in their heat with the fastest 100 split (52.64) among teammates Amanda Weir, Lia Neal and Allison Schmitt.

"It just made the first swim of the meet a lot more fun than it normally is," Ledecky said. "I think that bodes well for the rest of the week."

Ledecky will try to sweep the 200, 400 and 800 freestyles for the first time since 1968 in her individual events.

Overall, the Australian quartet of Madison Wilson, Brittany Elmslie and sisters Bronte and Cate Campbell qualified first in 3:32.39. The United States was next in 3:33.59, and Canada took third in 3:33.84. The Aussies are the defending Olympic champions, while the Americans haven't won gold since 1996.

In the 100 breast, Brazilian fans cheered wildly for Felipe Franca, who qualified third-fastest. Also moving on to the late-night semifinals were Americans Kevin Cordes (fourth) and Cody Miller (fifth).

Peaty wasn't full of adrenaline until he walked on deck.

"You can either be shy of the arena or you can take advantage of it," he said. "Going down that first 50, I knew it was fast, and I knew I turned fast. It wasn't until 25 meters to go, the crowd started to roar. I knew there were no Brazilians in this race, so they've got to be shouting for something."

Hosszu is seeking the first Olympic gold medal of her career, having long dominated at the world championships. She finished fourth in the 400 IM four years ago in London.

In the men's 400 IM, Chase Kalisz of the United States had the top time of 4:08.12. Japanese teammates Daiya Seto and Kosuke Hagino were second and third, while American Jay Litherland was fourth in making the final.

The grueling event featuring all four strokes — butterfly, backstroke, breaststroke and freestyle — was missing its two biggest stars. Michael Phelps, the world record holder and two-time Olympic champion, dropped the race for his fifth games and defending champion Ryan Lochte didn't qualify at the U.S. trials.

Sarah Sjostrom of Sweden qualified fastest in the 100 butterfly heats with a time of 56.26. American Dana Vollmer, the silver medalist in London, was second in 56.56.

A heat of the 400 freestyle included defending Olympic champion Sun Yang of China and 2012 silver medalist Park Tae-hwan of South Korea, both having served doping suspensions. Park finished 10th and didn't advance to the final.

Sun had the fourth-quickest time of 3:44.23, behind top qualifier Conor Dwyer of the U.S. (3:43.42), Mack Horton of Australia and Gabriele Detti of Italy.

In 2014, Sun served a three-month suspension for using a banned stimulant.

Horton was asked about a reported incident between the two at the practice pool earlier in the week. The Aussie said Sun "splashed me to say hello, and I didn't respond because I don't have time for drug cheats."

In another 400 free heat, Miguel Duran Navia of Spain dived in too early, triggering an automatic disqualification.

He slapped the water and hung his head on the lane line, realizing he was out of his only individual event. Duran appeared emotional as he picked up his clothing and walked off the deck while fans applauded in sympathy.

Moments later, Duran reappeared and took his position on the starting block, having been given a reprieve.

He said he heard a sound from the stands that caused him to false start.

Duran finished seventh in his heat with a time that was five seconds worse than his personal best and failed to advance to the final.

"I was very nervous, my mind wasn't where it was supposed to be," he said.