Singapore –

Leon Marchand smashed the 200-meter individual medley world record in Singapore on Wednesday with another breathtaking swim while Australia and the United States celebrated more gold.

Racing in the semifinals at the world championships, the Frenchman clocked 1 minute, 52.69 seconds to wipe more than a second off the previous record of 1:54.00 set by Ryan Lochte in 2011.

Marchand beat Michael Phelps' long-standing 400-meter medley record at the world championships in Japan two years ago.

"Actually, I can't believe it right now," said Marchand, who won four individual golds in front of his home fans at the Paris Olympics a year ago and was the face of the Games.

"I knew I was going to be close to my PB (personal best), because I felt really good today and preparation has been pretty good.

"But 1:52 is unbelievable for me."

Marchand, 23, took an extended break from swimming after Paris and only returned to competition in May.

He is focusing on the individual medley events in Singapore and had said he was gunning for Lochte's record.

Marchand got his world championships campaign underway on Wednesday morning, clocking a time of 1:57.63 in the heats.

He said he would go all-out for the record in the semifinals rather than conserve his energy for Thursday's final.

In Wednesday's finals, Tunisia's Ahmed Jaouadi won the 800 freestyle thanks to a perfectly timed attack.

Jaouadi made his move midway through the race and eased away to touch the wall in 7 minutes, 36.88 seconds, ahead of German pair Sven Schwarz (7:39.96) and 400 champion Lukas Maertens (7:40.19).

Jaouadi, who finished fourth at last year's Paris Olympics, clocked the third-fastest time ever.

"At some point I saw that the rhythm wasn't that fast so I decided to just go and make the move," the 20-year-old said.

Ireland's Olympic champion Daniel Wiffen finished last.

Australia's Olympic champion Mollie O'Callaghan was just as emphatic in romping to victory in the women's 200 freestyle in 1:53.48.

China's Li Bingjie (1:54.52) was second and Claire Weinstein of the United States (1:54.67) finished third.

O'Callaghan said it had been a "whirlwind."

Since winning gold at the Paris Games, O'Callaghan has been injured and had some badly needed time to recharge.

She has spoken about struggling to live up to the hype since Paris.

"I know it's been hard for a lot of people to come back after the Olympics but very grateful to have a good support team behind me to get me through this mentally and physically," the 21-year-old said.

Luca Urlando of the United States celebrated the biggest title of his career with gold in the 200 butterfly in 1:51.87.

Italy's Simone Cerasuolo pipped Russian Kirill Prigoda to 50 breaststroke gold, touching home in 26.54 seconds.

A "neutral athletes" team made up of Russian swimmers won the mixed 4x100 medley relay, finishing ahead of China and Canada.

After four days of the championships, Australia leads the medal table with four golds, with the United States second with three and Germany and Canada each with two.