GWANGJU, South Korea (AP) — After an unexpected assist from her coach, Katie Ledecky opened the world championships with a dominating performance in the preliminaries of the 400-meter freestyle Sunday.

The 22-year-old American led the qualifying in a time of 4 minutes, 1.84 seconds — 0.58 seconds faster than the second-quickest qualifier, Ariarne Titmus of Australia.

"That's right where I wanted to be," said Ledecky, who set the world record of 3:56.46 at the Rio de Janeiro Olympics three years ago.

Ledecky heads into the evening final as the favorite to win a record fourth consecutive world title in the event.

"It's going to be a great race," she said.

There was a brief delay before the last heat that included Ledecky and teammate Leah Smith, who qualified fifth. The swimmers stepped away from the blocks and sat down in chairs waiting.

The cap of a Canadian swimmer in the previous heat had come off and was floating in the pool. Ledecky said her coach Greg Meehan retrieved it, allowing the heat to go off.

Ledecky plans to swim four individual events and likely relays during the eight-day competition at Nambu University Municipal Aquatic Center.

China's Sun Yang led the men's 400 free prelims in 3:44.10 — 0.21 seconds ahead of Danas Rapsys of Lithuania — in pursuit of his fourth straight world title in the event.

Sun's presence has drawn the ire of some swimmers, including rival Mack Horton of Australia.

Sun is facing alleged doping rule violations that risk a ban from the 2020 Tokyo Olympics and he has requested a public trial at the Court of Arbitration for Sport in September to defend himself.

The World Anti-Doping Agency is challenging a decision by FINA, swimming's world governing body, merely to warn him over incidents during a doping control team's attempts to take blood and urine samples at his home in China last September, while allowing him to continue competing.

Horton advanced to the final with the fifth-fastest time after not qualifying at the Australian trials and joining the team as a discretionary pick.

"Trials probably wasn't a strong reflection of how I've been training," Horton said.

Gabriele Detti of Italy qualified fourth.

"He is the best and I try to beat him," he said of Sun's presence.

American Caeleb Dressel, who tied Michael Phelps' record of seven golds at a single worlds in 2017, tied with Andrii Govorov of Ukraine as the fastest qualifiers in the 50 butterfly. They each touched in 22.84 seconds.

World record holder Adam Peaty cruised into the semifinals of the 100 breaststroke in 57.59. He was 1.28 seconds ahead of second-quickest qualifier Ilya Shymanovich of Belarus.

In the women's 200 individual medley, Katinka Hosszu of Hungary cruised into the finals with the fastest time of 2:07.02. She was 2.43 seconds ahead of China's Ye Shiwen, second-quickest overall.

American Melanie Margalis advanced in third.

Sweden's Sarah Sjostrom led the way in the 100 butterfly in 56.45 chasing her third straight world title. Emma McKeon of Australia was second in 56.90.

"To be on that top level in one event for so long is very cool," Sjostrom said. "I always get better and better the more races I get."

In the relays, Australia was fastest in the women's 4x100 free relay with the U.S. women fourth, while the United States topped the men's 4x100 free.

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