Leon Marchand smashed the 200 meters 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's long-standing 400 meters 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 under way 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 meters 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 400m 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," said the 20-year-old. 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 meters freestyle in 1:53.48. Second was China's Li Bingjie (1:54.52) and third Claire Weinstein of the United States (1:54.67). 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," said the 21-year-old. Luca Urlando of the United States celebrated the biggest title of his career with gold in the 200 meters butterfly in 1:51.87. Italy's Simone Cerasuolo pipped Russian Kirill Prigoda to 50 meters breaststroke gold, touching home in 26.54 seconds. A "neutral athletes" team made up of Russian swimmers won the mixed 4x100m medley relay, finishing ahead of China and Canada. After four days of the championships, Australia lead the medals table with four golds, with the United States on three and Germany and Canada two. Summer McIntosh of Canada will be looking to make it three golds for herself in as many races on Thursday when she goes in the 200 meters butterfly. Also in the final is China's 12-year-old schoolgirl Yu Zidi, as she attempts to bag a historic medal on her debut competition on the world stage.