Gretchen Walsh sets a world record and Katie Ledecky secures her 4th trip to Olympics at U.S. trials

Walsh will compete in the finals for 100-meter butterfly on Sunday, where she will try to secure a spot on her first Olympic team.

Gretchen Walsh set a world record in the women's 100-meter butterfly Saturday night, posting a time of 55.18 seconds in a semifinal heat at the U.S. Olympic swimming trials.

Walsh was more than a half-second under world-record pace at the turn and finished strong to eclipse the mark of 55.48 set by Sweden's Sarah Sjöström at the 2016 Rio de Janeiro Olympics.

She held her hand over her mouth as she looked at the scoreboard in disbelief, a “WR” beside her name.

The 21-year-old Walsh, a native of Nashville, Tennessee, who competes for the University of Virginia, will return for the finals Sunday night looking to claim a spot on her first Olympic team.

Already making the Paris squad is Katie Ledecky, who secured her fourth trip to the Olympics in the women's 400-meter freestyle.

Cheered on by a crowd of 20,689 at the home of the NFL's Indianapolis Colts, Ledecky touched the wall in 3 minutes, 58.35 seconds.

Katie Ledecky made her fourth Olympic team after clinching a spot on Team USA in the women's 400m freestyle.

She improved on her time of 3:59.99 in the morning preliminaries and set herself up to make a run at another gold against a loaded field at the Paris Games. Australia's Ariarne Titmus is the defending Olympic champion and world-record holder (3:55.38), with Canadian phenom Summer McIntosh also in the mix.

The 27-year-old Ledecky is set to swim four events at the trials, all of them freestyle events ranging from 200 to 1,500 meters. She already has six individual gold medals — more than any female swimmer in Olympic history.

The expected second spot on the Olympic team will go to Paige Madden, the runner-up behind Ledecky at 4:02.08.