Saturday, 11 August 2018:

ARIARNE TITMUS of Australia smiles following victory in the Women's 400m Freestyle Final on day six of the Gold Coast 2018 Commonwealth Games at Optus Aquatic Centre in the Gold Coast, Australia.

The message from Australia's Ariarne Titmus to American great Katie Ledecky ahead of Tokyo 2020 is clear .

Titmus, 17, laid down the gauntlet to world and Olympic champion Ledecky by becoming only the third woman to crack the four minute barrier, clocking 3:59.66 to eclipse her own national mark and claim 400m Pan Pacs silver in Tokyo.

But it seems Ledecky has her own for the teenager after their epic Pan Pacs 400m freestyle clash on Saturday: bring it on.

Ledecky was on world record pace before being forced to hold out Titmus and claim gold in 3:58.50 on the second last night of the four-day meet.

Multiple world record holder Ledecky has been unstoppable in distance freestyle since first bursting onto the scene as a 15-year-old at the 2012 Olympics.

But Ledecky, now 21, is suddenly looking over her shoulder before Tokyo 2020 after Brisbane schoolgirl Titmus announced her arrival.

Titmus said the American was now firmly in her sights.

"That's the goal to be up there with her and hopefully she'll enjoy having someone to race," she said.

"She hasn't had anyone to race for a long time, so I'm getting closer, so it should be good."

Ledecky welcomed the challenge.

"It's exciting for me to see that (from Titmus). I was the only one under four minutes in a textile suit for a few years now," she said.

"I think it's exciting for me to see how I've put the standard out there and I know that a lot of girls are chasing that."

Titmus may have only just cracked four minutes but Ledecky has now shattered the barrier 15 times.

However, Titmus was just stoked to be close enough to get a good look at the American great with daylight usually separating her rivals.

"It was great being that close to Katie. I could try and see where I was up against her," Titmus said.

Titmus broke her previous national record of 4:00.93 to enter an elite swimming club.

AAP