Australia's Ariarne Titmus landed the first blow in her Olympic duel with US great Katie Ledecky on Monday, snatching 400m freestyle gold with the second-fastest time in history.

The young challenger finished strongly to leave defending champion Ledecky in her wake, touching in 3min 56.69sec, the second-fastest time ever in the event.

Ledecky -- who set the world record in Rio five years ago -- hit the wall for silver in 3:57.36, with China's Li Bingjie a distant third in 4:01.08.

Titmus was elated but composed after the "surreal" win in one of the most anticipated encounters of the entire Tokyo Olympics.

"It's probably the biggest thing you could pull off in your sporting career, I'm over the moon," the 20-year-old said.

"I don't really feel the hype, that's more you guys (media), I just try to race the best that I can and ignore everything else - honestly I thought I'd be more nervous coming into this."

Ledecky offered no excuses after losing an individual Olympic final for the first time in her career.

"I fought tooth and nail and that's all you can ask for," she said.

"I didn't feel like I died or fell off, she just had a faster last 50m or 75m and got her hand on the wall first"

Titmus' mission in Tokyo is to dethrone Ledecky in three individual events -- 200m, 400m and 800m freestyle -- and potentially the 4x200m relay.

Her coach Dean Boxall did not share his charge's calm demeanour after her successful start, with footage of him gyrating his hips and hurling his facemask into the air quickly going viral on social media.

Titmus said she saw Boxall "bawling his eyes out" as she received her medal in the spectator-free Tokyo Aquatic Centre.

'Smart race'

Ledecky, 24, who is also gunning for the 1500m freestyle, will have ample opportunities to avenge the loss and add to her Olympic medal tally, which now stands at five gold and two silver.

"I've got a lot of racing to do so I'm just going to try to use this for momentum," she said.

Titmus has been snapping at the American's heels since ousting her as world champion in 2019 and Ledecky, who is used to surging clear at the finish, praised her rival's "awesome" swim.

"She definitely swam a really smart race. She was really controlled up front," she said.

"I felt pretty smooth and strong going out and flipped at the 300 and it was like 'oh, she's right there'."

Titmus said she was proud to beat such a strong finisher.

"At the 200, I was a little bit worried... I just had to trust myself. I tried to stay as composed as I could and use the speed that I have," Titmus said.

"To pull it off in the backend against someone who has an amazing second half of her race, I'm really proud of that."

Titmus said she thanked Ledecky after the race for inspiring her, saying she was relishing their upcoming encounters.

"I wouldn't be here without her, she's set this amazing standard in middle-distance freestyle for girls and if I didn't have someone like her to chase I definitely wouldn't be swimming the way I am," the Australian said. 

"I'm really grateful to her and now it's good to have someone that I can race all the time. It's super fun. Now I can just have fun out there -- being in a battle is the best thing."