Ariarne Titmus isn't sure about the motive of American swim legend Katie Ledecky, who tasted success in their entree to a main course of the Paris Olympics.

And nor does the Australian ace really care after racing Ledecky in a women's 400m freestyle heat on Saturday morning.

Titmus led for 350 metres before Ledecky produced a swift last lap to finish first and secure lane four for Saturday night's hyped final.

Was the American sending Titmus a message?

'Maybe. I don't know,' Titmus said.

'I don't try to look too much into heat swims.

'But she has got lane four, I've got lane five, it doesn't really matter.

'Now it's all about who can do it tonight.'

Ledecky clocked four minutes 02.19 seconds with Titmus next-best in 4:02.46.

Canadian teen Summer McIntosh (4:02.65) swam in a different heat and was fourth-fastest into the medal race while Australia's Jamie Perkins (4:03.30, fifth-ranked) also advanced.

While Titmus was unsure of any Ledecky message, Australia's dominant women's 4x100m freestyle relay team sent an ominous signal to rivals in their heat.

Australia, world record holders and seeking a fourth consecutive Olympic gold medal in the event, rested top guns Mollie O'Callaghan and Shayna Jack.

The Dolphins deployed Games debutant Olivia Wunsch, four-time Olympian Bronte Campbell, Meg Harris and Emma McKeon and comfortably topped qualifying times in 3:31.57, clear of the United States (3:33.29).

In the men's relay equivalent, Australia's Jack Cartwright, William Yang, Flynn Southam and Kyle Chalmers won their heat which included the USA's gold medal favourites.

Australia (3:12.25) progressed to the final with the second-fastest time behind China (3:11.62).

Before the relay heats, Australia's most successful Olympian McKeon cruised into the women's 100m butterfly semi-finals.