So this is what happens when the world's fastest female swimmer trains alongside the world's fastest man.

Still coming to grips with a new stroke, Libby Trickett handed her 100m freestyle world mark a hell of a scare at the world championships trials in Sydney on Thursday night.

Instead of a post-Olympics hangover, Trickett (52.99 seconds) recorded the second fastest swim in the event's history - only 0.11 off her mark.

The performance puts her on track to defend her world title in Rome this July and showed her move south could prove a very wise one.

She headed down from Brisbane to Sydney following the Beijing Olympics and has already modelled her stroke on speed merchant Eamon Sullivan.

When there is no woman quicker than you in the world, you have to find new targets. And that is exactly what Trickett did, being the only woman in Grant Stoelwinder's squad that includes Sullivan and Andrew Lauterstein.

On chasing down Sullivan, she joked: "He better watch out".

That very man showed the Stoelwinder stable was firing on all cylinders by posting a sizzling semi-final swim in the 100m freestyle.

His time of 48.24s was particularly impressive coming just two months after hip surgery. He felt he still had another gear before taking on his main title contender Matt Targett (48.79) in Friday's final.

Sullivan admitted to being impressed by Trickett's performances but not in any way surprised.

"There is always that change over between coaches and she was so keen to pick up new things and try different things," he said.

"It is great to see and it is obviously working for her."

However there was mixed news for Stoelwinder's squad in the men's 50m butterfly. Lauterstein (23.34) edged out Targett (23.49) to ruin Geoff Huegill's (24.01) fairytale comeback, the Sydney and Athens Olympian finishing fifth.

The weight-loss star's original goal was to make next year's Commonwealth Games and he now has another 12 months to find some extra speed to achieve that.

In Leisel Jones' absence, Sally Foster (2:25.88) showed some serious guts to pull herself together and win the 200m breaststroke despite vomiting after the 100m freestyle.

Ashley Delaney (1:55.82) carved more than half a second off teammate Hayden Stoeckel's Commonwealth record in the 200m backstroke.

Emily Seebohm (59.85) captured the women's 100m backstroke while Christian Sprenger (1:00.64) and Brenton Rickard deadheated in the men's 100m breaststroke.

Jess Schipper (57.65) showed her new partnership with Trickett's old coach Stephan Widmer was also working by topping the timesheets for Friday's 100m butterfly final.

Tom Wald
AAP