TOKYO (AP) — Katie Ledecky opened her Olympic program as the top qualifier in the 400-meter freestyle, setting up a showdown with Ariarne Titmus of Australia as the American star tries to defend her title.

Ledecky advanced with a time of 4:00:45 on Sunday night. Titmus was third-quickest in 4:01.66. The first of their multiple showdowns in Tokyo comes in the final on Monday morning.

Ledecky will be looking to carry forward the momentum her teammates built on the first day of the meet, when the U.S. earned six of 12 medals Sunday.

But she can expect a fight from Titmus.

Ledecky won the 400 free with a world-record time five years ago in Rio. Titmus, the new star in Australian swimming, took down Ledecky at the 2019 world championships. She handed the American her first loss in the event at a major international meet, overtaking Ledecky in the final 50 meters to win by a full second.

China’s Li Bingjie qualified second in 4:01.57. Also making the final was American Paige Madden.

Ryan Murphy has some work to do if the American is going to defend his title in the 100 backstroke. He qualified eighth in 53.22 seconds for the 16-man semifinals. Kliment Kolesnikov of the Russian Olympic Committee led the way in 52.15. Also moving on were Mitch Larkin of Australia, Ryosuke Irie of Japan and ROC swimmer Evgeny Rylov.

“Luckily, I was in the last heat so I could see how fast the guys went,” Murphy said. “I just concentrated on my race based off that.”

In the women’s 100 breaststroke, South Africa’s Tatjana Schoemaker led the way in an Olympic-record time of 1:04.82.

Lydia Jacoby, the first U.S. Olympic swimmer from Alaska, was second at 1:05.52.

“To have my whole state of Alaska backing me, I know they’re up at 2:30 a.m. watching me, it’s been great,” she said.

American Lilly King, the defending gold medalist and world recordholder, was third-quickest in 1:05.55.

“I’m just kind of riding the wave I’ve been riding the last five years and hopefully it will all turn out all right,” King said.

Yulia Efimova of the Russian Olympic Committee, the silver medalist in Rio, advanced to the semifinals in eighth.

South Korea’s Sunwoo Hwang was the fastest qualifier in the men’s 200 freestyle in 1:44.96. Also advancing to the semifinals were 16-year-old David Popovici of Romania, Duncan Scott of Britain and Americans Townley Haas and Kieran Smith, who earned a bronze medal earlier Sunday in the 400 free.

Kaylee McKeown of Australia topped the 100 backstroke qualifying with a time of 57.88, capping the third straight heat in which the Olympic record was lowered. Also advancing were Americans Regan Smith in second and Rhyan White in sixth, and Kylie Masse of Canada in third.