FUKUOKA, Japan (Reuters) -French youngster Leon Marchand produced a dazzling performance to shatter American great Michael Phelps' long-standing 400 metres individual medley record on the opening day of swimming events at the World Aquatics Championships on Sunday.

Phelps' mark of 4:03.84 set at the 2008 Beijing Olympics was his last remaining individual world record but it fell to the 21-year-old Marchand who started superbly and clocked 4:02.50 to beat Carson Foster who took silver.

"That was insane, one of the most painful things I've done. It was amazing to do it here and the time is crazy," Marchand, who is coached by Phelps' former mentor Bob Bowman said.

"The best is yet to come," added Marchand, who collected his medal at the victory ceremony from Phelps.

There was cheer for home fans at the Marine Messe Fukuoka Hall as Japan's Daiya Seto took the bronze finishing more than six seconds behind Marchand.

Earlier, Olympic champion Ariarne Titmus set a world record by blazing to the finish line in a battle royale for the women's 400m freestyle title on a great day in the pool for Australia.

Titmus eclipsed the mark of 16-year-old Canadian prodigy Summer McIntosh set at her national trials earlier this year, finishing in three minutes, 55.38 - a stunning 3.35 seconds ahead of silver medallist and 2022 champion Katie Ledecky.

Victory means the 22-year-old has not lost a 400m freestyle race she has competed in for five years.

"I've put in a lot of work the past six weeks that I've been really happy with and took a lot away from trials," Titmus said. "I swam the way I wanted to, but knew I had enough time to turn that around and came here tonight and tried to be fearless.

"Summer and Katie are both class acts and there's hardly any faults in their racing. I knew the only way was to try and take the win... I guess (it was) whoever had the most fight in the end and whoever had the best left in the tank."

The clash between the trio had already generated "race of the century" hype, 20 years after Ian Thorpe beat Pieter van den Hoogenband and Phelps at the 2004 Athens Olympics.

Although it was not as close a contest as fans would have liked, Titmus's blistering effort laid down a marker for next year's Paris Games.

New Zealand's Erika Fairweather edged out a disappointed McIntosh to bag bronze, finishing 4.21 seconds behind Titmus.

Earlier, Australia's Sam Short won the men's 400m freestyle race a year after his compatriot Elijah Winnington prevailed.

Short, 19, crossed the line in three minutes, 40.68 seconds to edge out Olympic champion Ahmed Hafnaoui of Tunisia by 0.02 seconds while Germany's Lukas Martens was 1.52 seconds behind the winner to take the bronze medal.

"I knew it was going to be a race of two with 100m left and he (Hafnaoui) was the defending Olympic champion," Short said.

"I had goosebumps thinking I was in the race with him and got my hand on the wall first... dream come true."

Winnington, who bagged the title in Budapest last year, was seventh out of eight competitors.

Australia's women's 4x100m freestyle relay team set a world record with a scorching time of 3:27.96 to beat the United States, who came home nearly four seconds slower while China were third.

The Dolphins' men's team then bagged the gold in 3:10.16 ahead of Italy and the U.S. to bring an end to the day's action.

The British men's team, pre-meet favourites for the crown, did not compete after being disqualified due to an early takeoff by Jacob Whittle in their heat in the morning.

(Reporting by Shrivathsa Sridhar in BengaluruEditing by Toby Davis)