Olympic champion Kaylee McKeown fired a warning shot to American arch-rival Regan Smith on Wednesday with the third-fastest 100m backstroke ever as she warms up for next month's world championships in Japan.
The 21-year narrowly missed her own 57.45 world best when she touched in 57.50 at the Australian trials in Melbourne ahead of 100m freestyle world champion Mollie O'Callaghan.
McKeown – who also holds the 200m backstroke world record – now owns the top three times in history.
"I felt really nice. I'm just doing the best I can to get myself in a good position for the world champs," said McKeown, who won the 200m medley on Tuesday.
"Without Molls (O'Callaghan) being there I don't think I would have swum as fast as I did. It's always great to have her alongside, I love it."
O'Callaghan's 58.42 also saw her qualify for the world championships in Fukuoka.
McKeown is also set to race the two other backstroke distances in Melbourne, along with the 200m freestyle.
Teenager Kai Taylor unexpectedly won the men's 200m freestyle, hitting the wall first in 1:46.25 ahead of Alex Graham.
The 19-year-old only finished ninth in the heats but got his chance when sprint king Kyle Chalmers, who qualified fastest, opted out.
Taylor's time was fractionally outside what he needed to qualify automatically but he booked a place on the 4x200m freestyle relay.
"It feels really good, especially after this morning when I was quite disappointed when I didn't have a great race," said Taylor. "But fortunately Kyle pulled out and I just tried to stay calm."
Chalmers decided to conserve his energy for the 100m that is his main event, coming second to American arch-rival Caeleb Dressel at the Tokyo Olympics after winning gold in Rio in 2016.
Issac Cooper won the men's 100m backstroke ahead of Bradley Woodward but his 53.46 was outside the automatic qualifying time. He could still make the plane at the discretion of national coach Rohan Taylor.
Former world champion Mitch Larkin missed the trials with injury.
Elsewhere, Abby Harkin took out the women's 100m breaststroke in 1:07.20 but she also missed the time needed for Japan. Australia's top swimmers over the distance, Chelsea Hodges and Jenna Strauch, were absent due to injury.