When Ariarne Titmus woke, she channelled her inner child.

"I was just a little girl that started swimming with a big dream," Titmus said after winning a famous Olympic gold medal.

"And I tried to think about that this morning when I woke up.

"You know, I just have to go out there and swim. That is why I am here.

"And to finally be able to achieve something I have dreamt of, I'm really, really happy."

Titmus was largely shuttered to the hype of her duel in the pool with American Katie Ledecky, the greatest female swimmer ever.

Monday's 400m freestyle final was a race that stopped two nations. And it stopped swimmers in their tracks at the Tokyo Olympics.

"Gave me goosebumps," said Australia's swim ace Kyle Chalmers, who watched Titmus' win from the warm-up pool.

Titmus is now trying to keep a lid on it. One gold medal over Ledecky, two to go.

The freestyle rivals square off again over 200m and 800m, and possibly come across each other in a 4x200m relay.

"It was really nice to be in the race with her," Titmus said of Ledecky, a five-time Olympic and 15-time world championship gold medallist.

"I was excited that to win this event I had to beat a great champion. And that makes it even more satisfying.

"She has done unbelievable things for this sport, especially middle-distance swimming, and I am very grateful."

The 20-year-old Tasmanian arrived at the pool with a strange sense of calm.

"I do feel pressure," she said.

"But I feel like I do a good job of eating it all up and using it.

"And I have been very relaxed at this meet, more relaxed than I thought I would be.

"I was way more nervous swimming at trials than I was tonight - and I was a bit worried."

The final became a match race for the ages. And the defeated Ledecky offered nothing but praise for her conqueror.

"She definitely swam a really smart race," the American said.

"She was really controlled up front."

But so was Ledecky.

"I felt smooth and strong," she said.

Ledecky was in her zone when she ominously sensed Titmus close the gap.

"I looked up at 300m, and she was right there," Ledecky said.

"So I knew it would be a battle to the end.

"I didn't feel like I died or really fell off.

"She just had a faster final 50 or 75m and got her hand to the wall first."

Titmus had no idea who had on.

"About the 350 (mark) we turned together and then on the last lap we were both breathing the opposite way," she said.

"So I had no idea where she was.

"I was just going for it and hoping that it was enough to get my hand on the wall first.

"I did not know that I was in front until I saw the scoreboard."

And that, Titmus said, was part of the thrill.

"That's exciting. If I was looking at her (while swimming) I think it wouldn't have been such a shock.

"Breathing the other way meant I got an actual surprise when I saw the scoreboard."

For Titmus, there's no point pondering the magnitude of her feat just yet.

"I don't think I will know what it means until I get back home," she said.

"I think about everyone back home at the moment doing it tough. I feel nice that I can put on a bit of a show for them and bring some joy to their lounge rooms.

"If people back home can enjoy this moment like I can, I am really happy I have brought some joy to the country during a tough time."