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

Phelps' mark of 4:03.84 set at the 2008 Beijing Olympics was his last remaining individual world record. But 21-year-old Marchand, who had a superb start, smashed it when he 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," said Marchand, who is coached by Phelps' former mentor Bob Bowman.

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

Fans at the Marine Messe Fukuoka Hall cheered as Japan's Daiya Seto finished more than six seconds behind Marchand to take home the bronze.

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 time of 16-year-old Canadian prodigy Summer McIntosh set at her national trials earlier this year, finishing in 3:55.38 - a stunning 3.35 seconds ahead of silver medalist 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," she noted.