Katie Ledecky sets up 400m showdown with champion Ariarne Titmus

Three years after Titmus took Ledecky's Olympic crown in a Tokyo thriller, the pair lined up next to each other on the blocks and were again neck-and-neck at a rocking La Defense Arena before Ledecky pulled away in the final lap.

Ledecky touched first in four minutes 2.19 seconds, nearly three-tenths of a second quicker than second-ranked Titmus.

Canadian wunderkind Summer McIntosh is also in the mix for what should be an enthralling 400 decider later on Saturday, having qualified fourth behind New Zealand's Erika Fairweather.

"That was good. It's good to walk out to a full stadium for a prelims race," seven-time Olympic champion Ledecky told reporters.

"It felt very similar to our Olympic trials a few weeks back, so I'm excited to see the finals atmosphere tonight as well and I'm happy to get a good first one on the board."

World record holder Titmus was satisfied to "blow out the cobwebs" and dismissed the idea that Ledecky's top ranking in the final was a psychological edge.

"I don't think so. It's a heats swim," said Titmus, also the Olympic 200 freestyle champion. "I tried to conserve as much as I could so I think that I'm happy."

China's Zhang Yufei, the Tokyo silver medallist, was fastest in the 100 metres butterfly heats with a time of 56.50 seconds, ahead of Japan's Mizuki Hirai (56.71) and American Torri Huske.

The U.S. world record holder Gretchen Walsh was also safely through to the evening's semi-finals along with Canada's defending champion Maggie MacNeil.

Doping suspicions have followed the Chinese team to the Paris pool after revelations in April that 23 of the nation's swimmers were cleared to compete at Tokyo after testing positive for a banned heart medication.

Zhang said she had been affected by the acrimony, being one of the swimmers reported by the New York Times and German broadcaster ARD to have tested positive.

"Now I come to participate in the Olympic Games, I'm very worried that my good friends look at me with coloured eyes (and) do not want to compete with me or watch my games," she said.

The World Anti-Doping Agency accepted the findings of a Chinese investigation that the test results were due to contamination from a hotel kitchen the team were staying at.

Britain's Adam Peaty, bidding for a third successive 100 breaststroke gold medal, qualified second fastest for the semi-finals behind the Netherlands' Caspar Corbeau.

"I'm just free and easy. Now, not too many nerves," said world record holder Peaty, who clocked 59.18. "That comes down to experience. I've been in this position a lot of times."

Peaty's major threat, the Chinese swimmer Qin Haiyang, had a relatively slow start, qualifying ninth in the heats.

Germany's rapidly improving Lukas Maertens has top rank for the men's 400 freestyle final after winning his heat in 3:44.13, a tenth of a second quicker than Brazil's Costa Guilherme.

Australia's former world champions Elijah Winnington and Sam Short are also expected to contend for the gold.

They qualified fourth and fifth respectively in the heats, while South Korea's reigning world champion Kim Woo-min was seventh through to the final.

Gold medal favourites Australia enter the women's 4x100m freestyle relay final as the top seeds after dominating their heat in 3:31.57.

Nearly two seconds slower, the United States are seeded second ahead of third-quickest China.

China, however, were fastest into the men's 4x100m freestyle final with a time of 3:11.62, leading Australia and Britain.

With Caeleb Dressel anchoring, the United States finished fourth quickest in the heats, nearly a second behind their Chinese rivals.