OMAHA—Regan Smith swam her first race at the Olympic trials on Sunday morning, reaching the semifinals of the women's 100-meter butterfly. While she was getting started in Omaha, one of her world records was broken on the other side of the planet.

Australia's Kaylee McKeown took down Smith's record in the 100 backstroke, clocking a time of 57.45 seconds in the finals at the Australian Olympic trials. That topped Smith's world record time of 57.57 set at the 2019 world championships. McKeown, 19, beat runner-up Emily Seebohm—a three-time Olympian—by more than a second.

Smith, of Lakeville, remains the world record holder in the 200 back. Sunday, the 19-year-old opened the U.S. Olympic trials with the 100 fly, qualifying for Sunday night's semifinals with the fifth-fastest time in the preliminaries.

The trials began Sunday morning with preliminary heats in five events. The first members of the U.S. swim team for the Tokyo Olympics will be named Sunday evening, when finals will be held in the men's and women's 400 individual medley and men's 400 freestyle.

Smith finished her 100 fly heat in 57.63 seconds, charging through the final 50 meters in 30.49, the third-fastest final 50 in the field. She was second in her heat to Torri Huske, a 19-year-old who has been swimming very fast this year. Huske tied her personal best with a time of 56.69.

2016 Olympian Kelsi Dahlia led the qualifying heats in 56.56.

Sunday night's 100 fly semifinals include the 16 fastest swimmers from the morning heats. Eight will move on to Monday's finals.

Smith said last week it's possible she will not swim the finals of the 100 fly if she advances. The preliminaries and semifinals of the 100 back also are Monday, and that race has one of the deepest and most talented fields at the Olympic trials. Unless she "is really in the mix'' in the 100 fly final, Smith said, she could scratch to conserve her energy during a schedule that also includes the 100 back, 200 back and 200 butterfly.

The finals of the 100 fly are the first race of Monday night's final, with the semis of the 100 back the last of the seven races that evening.