American Katie Ledecky has fired the first shot in her epic rivalry with Australia’s golden girl of the pool Ariarne Titmus, beating her in the 400m freestyle heat by 0.27 seconds.

In the reverse of three years ago in Tokyo, Titmus led for much of the heat only to be mowed down by the powerful finishing American.

The loss means little, though, as Titmus was more than fast enough, 4:02.46, to book a spot in the medal race.

“I don’t try to look too much into heat swims,” Titmus said.

“But (Katie) has got lane four, I’ve got lane five, it doesn’t really matter.

“Now it’s all about who can do it tonight.”

Canadian teen Summer McIntosh (4:02.65) swam in the different heat and was fourth-fastest qualifier for the medal race with New Zealand’s Erika Fairweather (4:02.55) third-quickest.

Elijah Winnington also gave Australians a reason to be excited about the men’s 400m freestyle, pushing German star Lukas Maertens to the final 25m to register the second-fastest time of the fastest heat.

An incredible third leg of the 4x100 metre freestyle saw the Australians go from sitting second behind Sweden to blowing the field out of the pool to finish fastest of the heat in a landslide.

The Swedes made an incredible start, not only sticking with Australia, but beating them, but with a seemingly endless list of champions, the Aussies bossed the last two legs and will go into the final as favourites.

Kyle Chalmers handed the Americans a reality check in the men’s relay, going toe-to-toe with Caeleb Dressel in the final leg to overcome the powerhouse nation.

The Chinese also looked very quick in their heat as a clear top three emerged.

Earlier on the opening day of competition at the Paris pool, Australia’s most decorated Olympian Emma McKeon cruised into the women’s 100m butterfly semi-finals.

Five-time gold medallist McKeon was fifth-fastest through the heats in 56.79 seconds, with her Dolphins teammate Alexandria Perkins (57.46) the eighth-quickest qualifier for the semis.

- With AP