Aussie favourite Ariarne Titmus has been pipped by American Katie Ledecky in a blockbuster 400m freestyle heat on the opening day at the Olympics.

Titmus, who won the event at the Tokyo Olympics, looked set to win, but her great rival came from behind to take the honours in the final 50metres.

Ledecky posted a time of 4:02.19, just in front of Titmus at 4:02.46, with the pair taking the fastest two times from the three heats.

Titmus, the world record holder and five-time gold medallist, will now swim in lane five in Sunday's final.

'Just having a look at Arnie at the end of that race, she actually didn't look that pushed,' Giaan Rooney said on Channel Nine.

'It didn't look comfortable in that last 100 (metres) but I also wonder if that was a bit of a play of ''I don't want to burn everything I have this morning, if you want to post that time you post that time, I don't need to''.'

The rivalry between Ledecky and Titmus was turned up a notch this week, with the American slyly snubbing her rival before their battle for Olympic gold.

Ledecky, the most successful female swimmer ever with seven Olympic gold medals and 21 world championship golds, refused to say Titmus' name during a packed press event in Paris. 

Until Saturday, the American superstar hadn't beaten the Aussie superfish in the 400m freestyle in five years - and couldn't bring herself to acknowledge Titmus personally, despite being her biggest threat to taking home the gold. 

'I like my chances, but yeah that's me and I feel like I'm prepared and ready to race and that's all you can ask for,' Ledecky said.

'So I'm gonna give it my best shot and it's obviously a great field, top to bottom. Lots of people that have a chance so I'm just gonna put up a great race in the morning and at night and see where that lands me.

'They're great athletes and I've had the chance to race them quite a few times over the years now, especially Summer (McIntosh) being in the US training in the US and we've raced a lot the last couple of years.

'It's always fun to race the best and we have the top three times ever in that race so that just makes for a great field, great race.

'Those two have continued to raise the game, raise my game. I know that I have to bring my best. I think they know that they have to bring their best.'

Another Aussie has also made the final, with Jamie Perkins qualifying fifth fastest.

New Zealander Erika Fairweather (third fastest qualifier) and Canadian McIntosh (fourth fastest) also both swam well, setting up a mouthwatering final for tomorrow.