Michael Phelps hailed Katie Ledecky as "the best female swimmer that we have ever seen" as the American looks to add to her haul of five Olympic golds in Tokyo.

Ledecky won the gold medal in the 800m freestyle race at London 2012 at the age of 15 and went on to scoop four golds in the Rio Games five years ago.

The 24-year-old will be aiming to defend her titles in the 200m, 400m and 800m individual freestyle races, as well as the 4x200m freestyle relay.

A new event has also been added in for the women at this year's Games, with the 1500m freestyle up for grabs.

Given her achievements so far, Ledecky may well have Phelps' all-time Olympic record of 23 gold medals in her sights.

Phelps, who also holds the record for the most gold medals in individual events (13) and Olympic medals in individual events (16), believes Ledecky already has to be considered one of the greats.

"Katie and I have known each other for a long time," the 36-year-old said in a Panasonic Instagram live interview. "She is hands down the best female swimmer that we've ever seen.

"I always say one thing for her is just be her. As long as she's being herself and preparing for what she has to do, everything else will happen how it's supposed to.

"She's somebody that understands what to do in this setting and she's going to go out there and have some fun and we're going to see a lot of fast times."

Phelps also suggested Ledecky, and other athletes competing for the top prizes in Tokyo, must go into "autopilot" in order to keep their composure when it counts.

He added: "You've done the hard work, now it's just time to let it all show.

"Go out there, have some smiles, have some laughs and perform.

"From [Athens] 2004 on, I feel I was almost on autopilot because the preparation was done. All I had to do was just get on the blocks and race."