Mollie O'Callaghan and David Popovici backed up from winning 200m freestyle gold with strong 100m heat swims Tuesday, as French hero Leon Marchand dived into the Olympic pool twice in his bid for more glory.

Dominant American great Katie Ledecky was also back in action at La Defense Arena, cruising through her 1,500 freestyle heat as she bids to defends her title.

Australia's O'Callaghan upset defending champion Ariarne Titmus to clinch the 200m crown on Monday, while Romanian teenager Popovici won by a fingertip in a frenetic men's race.

They both returned barely 12 hours later, with Australian two-time world champion O'Callaghan fifth fastest through the heats behind pace-setting world record holder Sarah Sjoestroem (52.99sec) and Hong Kong's Siobhan Haughey.

Popovici -- also a former world champion -- was third best after timesheet-topping American Jack Alexy (47.57), with Chinese world record holder Pan Zhanle just scrapping through in joint 13th.

With reigning champion Emma McKeon failing to qualify at the Australian trials, O'Callaghan has a golden opportunity to secure the revered 100-200 double.

She has not lost a 100 free in a major final since 2021, although Haughey came to Paris as the top seed after her 52.02 last year made her fastest since McKeon's 51.96 at the Tokyo Olympics.

The men's 100m is also without a defending champion after Caeleb Dressel's failure to make the grade at the US trials, leaving it wide open.

After standing for 13 years, Cesar Cielo's world record was finally topped by Popovici in 2022.

But less than two years later Pan bettered it again, clocking 46.80.

Pan, though, seemed out-of-sorts and failed to make it through the 200m heats, only surviving the 100m by 0.06.

- Good shape -

Dressel's long-time rival Kyle Chalmers -- who won gold in Rio and silver behind the American in Tokyo -- was sixth fastest and fancies his chances again.

"I feel like I'm in as good a shape as I could possibly be in for Olympic Games, definitely better than what I was in Tokyo," said the Australian.

"Every race I enter I back myself. I think I got to have the confidence, and belief in myself that I can."

Marchand was again greeted by huge roars after he won France's first pool gold in 12 years in the 400m medley on Sunday.

His brutal schedule meant he was up in both the 200m butterfly and 200 breaststroke heats.

He clocked 1:55.26 to be sixth fastest in the butterfly, then posted 2:09.55 in the breaststroke behind only South Korea's Cho Sung-Jae and Australian defending champion Zac Stubblety-Cook.

"I'm using all the energy from the stadium and just trying to swim as fast as possible in every race," said Marchand.

"Tonight will be very exciting for me," he added, looking ahead to the semi-finals.

He faces a huge butterfly hurdle in Hungarian Olympic champion and world record holder Kristof Milak, who topped the times in 1:53.92

Milak broke Michael Phelps' 10-year old world record in 2019 and then smashed it again in 2022, but took a break last year.

In his absence, Marchand won the 2023 world title and is the third-fastest swimmer in history behind Phelps and Milak.

A key casualty in the heats was Tokyo silver medallist Tomoru Honda of Japan.

On paper, Marchand also faces a stiff breaststroke test from China's Qin Haiyang, who won all three gold at the 2023 world championships and shattered Stubblety-Cook's world record.

But Qin -- reportedly among 23 Chinese swimmers who failed doping tests before the Tokyo Games -- only came 15th, after fading to finish seventh in the 100m final.

A relaxed Ledecky, who has set the last six world records, was fastest by nearly four seconds in the 1,500m, touching in 15:47.43 ahead of Italian Simona Quadarella as she targets an 11th Olympic medal.

Britain qualified first for the men's 4x200m relay final later Tuesday ahead of the United States.