Paris, France - American great Katie Ledecky edged defending champion Ariarne Titmus to be quickest into the Olympic 400m freestyle final Saturday, setting the scene for a titanic battle for gold.

The veteran, who was stunned by the Australian in Tokyo three years ago, touched in 4 mins 02.19 secs at a packed La Defense Arena ahead of an evening final on day one of the nine-day meet.

Racing in the same heat, Titmus went out hard and led through 300m before Ledecky came storming home to win by a fingertip.

Titmus hit the wall in 4:02.46, well outside her 3:55.38 world record, as both swimmers conserved energy.

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New Zealand rising star and current world champion Erika Fairweather was third quickest in 4:02.55, a fraction clear of Canadian Summer McIntosh, who snatched the world record last year before Titmus took it back.

"Good to walk out to a full stadium for our prelims race," said Ledecky, who will also swim the 800m and 1500m in Paris but has dropped the 200m.

"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 just happy to get a good first one under the belt."

Titmus said the race helped "blow the cobwebs out" and denied coming second gave Ledecky any psychological advantage.

"I don't think so. It's a heats swim," said the Australian, who broke the 200m freestyle world record and swam the second-quickest 400m ever at the Australian trials last month.

"The only job for me was to get in the middle lanes [for the final], and I tried to conserve as much as I could."

In the day's other heats, American world record holder Gretchen Walsh was fourth fastest into the women's 100m butterfly semi-finals in 56.75.

The 21-year-old is at her first Olympics after shattering Sarah Sjoestroem's eight-year-old world best at the US trials to position her as the woman to beat.

But it was Tokyo silver medallist Zhang Yufei, also allegedly among the swimmers caught up in the recent Chinese doping scandal, who touched first in 56.50.

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Japan's Mizuki Hirai was second fastest ahead of American Torri Huske. Australia's Emma McKeon, the queen of the pool in Tokyo with seven medals, came fifth, and Canadian defending champion Maggie McNeil seventh.

Meanwhile, Australia's world-leading women's 4x100m freestyle relay team stormed through their heat in 3:31.57, nearly two seconds clear of the US.

Undefeated at the Olympics since 2012, they have owned the world record since 2014.

China was fastest into the men's 4x100m freestyle relay final ahead of Australia and Britain.