The best female backstrokers of all time, McKeown and Smith have a storied rivalry that has ebbed and flowed, with world records changing hands between them. They own the 15 fastest times ever swam over 100m and are similarly dominant in the 200m.

At La Defense Arena, American world record holder Smith laid down a Paris marker, touching in 58.45secs to be second best into Monday evening's 100m semi-finals. Australian defending champion McKeown, in a different heat, hit the wall in a composed 58.48, but it was American Katharine Berkoff who topped the timesheets in 57.99. 

Berkoff is one of only three other women, besides Smith and McKeown, to break 58 seconds, as is Canadian Kylie Masse, the Tokyo silver medallist who was fourth fastest (59.06).

The scene was set for the latest chapter of the McKeown-Smith duel at their respective Olympic trials. The Australian, then the world record holder, swam the second-fastest time ever to fire a warning shot. 

Though Smith responded with a sizzling 57.13 to take back a world record she first broke in 2019 before a loss of confidence saw her play second fiddle at the Tokyo Games to her rival, who won the 100m-200m double.

McIntosh, who already has a silver medal behind Ariarne Titmus in the 400m freestyle, is the raging favourite in the medley with her own 4:24.38 world record, set in May, potentially on the line. Still only 17, she cruised through her heat in 4:37.35, easing up in the final 50m ahead of Monday evening's final. American pair Emma Weyant (4:36.27) and Katie Grimes (4:37.24) qualified quickest.

"I was just trying to get the heat out of the way and try to get a good time heading into the final. So that goal was accomplished, I'm excited," said McIntosh, who will also race the 200m medley and 100m butterfly.

Not 100 percent

Fellow teenager Grimes said she was taking it easy.

"Got a good lead on the first 100, so there was no reason to keep pushing it. So yeah, I'm feeling good going in tonight," she said.

With 2023 world champion Ahmed Hafnaoui not at the Olympics and Germany's Lukas Maertens -- who won 400m gold on Saturday— focusing on shorter distances, the men's 800m freestyle is wide open.

Current world champion Daniel Wiffen surged into the final in 7:41.53, ahead of Tunisia's Ahmed Jaouadi.

"Comfortable, it just wasn't 100 percent, that's all I'd say. I actually thought I was going to be a bit faster," said Ireland's Wiffen.

American distance star Bobby Finke, the Tokyo gold medallist, and Australia's Elijah Winnington, fresh from winning 400m silver, also progressed. But Australia's Sam Short, seen as a medal contender, came ninth and missed out.

In other Monday finals, world record holder Titmus will defend her 200m freestyle title with teammate Mollie O'Callaghan breathing down her neck. Titmus bettered O'Callaghan's world record last month, making the pair overwhelming favourites.

Romanian David Popovici was fastest into the men's 200m freestyle decider while South Africa's Tatjana Smith will face a challenge from world record holder Lilly King and China's Tang Qianting in the women's 100m breaststroke. Italian world record holder Thomas Ceccon is favoured to take out the 100m backstroke gold medal with American Ryan Murphy and China's Xu Jiayu in the hunt.