American great Katie Ledecky edged defending champion Ariarne Titmus to be quickest into the Olympic 400m freestyle final on 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 minutes 02.19 seconds 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.

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." (AFP)