Ariarne Titmus, Kaylee McKeown come close to world records at Olympic trials

Ariarne Titmus took an experimental approach and Kaylee McKeown just tried not to get disqualified.

Yet both swim aces threatened world records only to fall just shy at Australia's Olympic selection trials on Monday night.

For Titmus, breaking her own 400m freestyle world record wasn't on her mind at the Brisbane Aquatic Centre.

But until fading over the last 10 metres, she was under record pace.

"I took it out and I was quite fearless," Titmus said after clocking three minutes 55.44 seconds, just missing her global record of 3:55.38 set last July

"Trials is a bit of a free shot for me.

"I don't have to worry about getting under the qualifying time so if I stuff up I have the luxury that I'd probably still be on the team.

"So it's about trying new things.

"And I think to win, you always have to try new things, you can't just expect to keep doing the same thing and expect to keep winning."

Lani Pallister finished second behind Titmus in 4:02.27, bettering the Olympic qualifying time set by Swimming Australia.

McKeown set a Commonwealth record in the 200m individual medley, a gruelling event she is adding to her program at the Paris Olympics.

The reigning Olympic 100m and 200m backstroke champion touched in 2.06.63. The world record of Hungary's Katinka Hosszu is 2:06.12 and has stood for almost nine years.

McKeown's time is the fastest in the event this year but, like Titmus, she wasn't targeting a record.

"When you're in the middle of a race, you kind of just switch off and I focus on trying to not get DQ'd (disqualified)," she said.

Australia's most successful Olympian Emma McKeon booked her spot at the Paris Games starting on July 26.

McKeon, who has already won five gold, two silver and four bronze Olympic medals, triumphed in the 100m butterfly.

"The pressure is always on at Olympic trials. It's just crazy," said McKeon, who has freestyle sprints to come later in the seven-day meet.

"Everyone is always going to be very nervous so I am glad I have got my first one out of the way."

McKeon clocked 56.85 while second placegetter Alex Perkins finished in 57.33, some 0.16 seconds outside of the automatic qualifying mark.

In the mens 400m freestyle final, Elijah Winnington, the 2022 world champion in the event, and Sam Short, the 2023 world champ, continued their rivalry with a stroke-for-stroke battle.

Winnington (3:43.26) pipped Short (3:43.90) with a last-gasp burst.

And in the men's 100m breaststroke, Sam Williamson's winning time of 58.80 ensured his ticket to Paris and Joshua Yong placed second in 59.48, one-hundredth of a second inside the qualifying time.