Marchand smashes Phelps' last remaining world record

France's Leon Marchand smashed Michael Phelps's last remaining world record on Sunday while Australia's Ariarne Titmus blew her 400m freestyle rivals out of the water at swimming's world championships.

Marchand clocked 4 minutes, 02.50 seconds to win the 400m individual medley, taking more than a second off Phelps's record.

Phelps' benchmark of 4:03.84 had stood since the 2008 Beijing Olympics and was the longest-held world record in swimming.

Marchand is set to be one of the faces of next year's Paris Olympics and he warned that "the best is yet to come".

"It was insane," Marchand said.

"That was one of the most painful things I did. That was amazing to do it here. The time is crazy."

Phelps's world record was one of three to fall on a dramatic opening night in the southern Japanese city of Fukuoka.

Titmus reclaimed her world record in the women's 400m freestyle, while Australia's women's 4x100m freestyle team also improved the mark they set two years ago.

Phelps, the most decorated Olympian of all time with a record 28 medals, was on hand to present Marchand with his gold.

Marchand said the American great was "impressed by the time" and told him he can continue to improve.

"He was commentating the race so it was pretty cool to see that," said Marchand, who is coached by Phelps' former mentor Bob Bowman.

American Carson Palmer finished second behind Marchand in a time of 4:06.56, followed by Japan's Daiya Seto in 4:09.41.

"Being the closest person to history was pretty cool, getting a front-row seat to that," said Palmer.

Titmus earlier dominated in one of the most hotly anticipated races of the championships, the women's 400m freestyle.

It pitted Olympic champion Titmus against Canada's world record-holder Summer McIntosh and American defending world champion Katie Ledecky.

Titmus made short work of her rivals, coming home in 3 minutes, 55.38 seconds to win the title and take back the world record that McIntosh had wrested from her in March.

Ledecky finished the race second in 3:58.73 but 16-year-old McIntosh could only place fourth, with New Zealand's Erika Fairweather taking third in 3:59.59.

Titmus said she "honestly did not think about getting the world record back".

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