Canadian teenager Summer McIntosh has smashed Ariarne Titmus's 400 metres freestyle world record, setting up keenly-anticipated showdowns at this year's world championships and 2024 Paris Olympics.

Competing on the opening night of Canada's national trials in Toronto, 16-year-old McIntosh clocked 3:56.08, shaving 0.32 seconds off the mark set by Australian star Titmus at her nationals in Adelaide last year.

"I'm not a crier but this world record, I didn't think this was possible tonight," said McIntosh, who finished fourth in the 400 free in her Olympic debut at Tokyo as a 14-year-old.

"It's absolutely incredible. I'm not an emotional person. But I was hit with so much emotion. Pure euphoria right now.

"Over the past few years I've put my life into this. To be the best I can be. To achieve something like this, it was very unexpected. It was never in my dreams to do this tonight or even a few years ago. This just blows my mind."

McIntosh's head coach Brent Arckey said the Toronto native was "one of the best racers I've ever seen".

"I'm not really sure what to say right now. I'm trying to hold it all in. But celebrating her. This is a special thing," he told CBC Sports.

The Canadian trials are being held to determine the swimmers who will compete at the world championships in Fukuoka, Japan in July.

McIntosh's emergence has set the scene for titanic world championship and Olympic 400m freestyle tussles with Titmus, the reigning Olympic champion, and American legend Katie Ledecky, another former world-record holder, the 2016 Olympic gold medallist and defending world champion.

with The AP