Australia's swimming squad know they will not have it all their own way at the Paris Olympics, with American Gretchen Walsh breaking the world record for the women's 100 metres butterfly.

A day after the Australian selection trials wrapped up, Walsh kicked off the US equivalent in Indianapolis with a bang, posting a time of 55.18 seconds in her semifinal.

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

Cheered on by a crowd of 20,689 at the home of the NFL's Indianapolis Colts, Walsh held her hand over her mouth as she looked at the scoreboard in disbelief to see "WR" beside her name.

"I definitely was feeling it," said Walsh, who considers herself a stronger swimmer in the short-course pool.

"I thought I was dying. I didn't know I was going that fast and, apparently, I took it out too fast."

The 21-year-old from Nashville will return for the finals on Monday, Australian time, looking to claim a spot on her first Olympic team.

Five-time Olympic gold medallist Emma McKeon won the women's 100m butterfly at the Australian selection trials in Brisbane with a time of 56.85.