Leon Marchand is focusing on the individual medley events in Singapore. (AP pic) SINGAPORE: Leon Marchand smashed the 200m individual medley world record in Singapore today with another breathtaking swim while Australia and the US celebrated more gold. Racing in the semi-finals at the World Championships, the 23-year-old Frenchman wiped more than a second off the previous record of 1:54.00s set by Ryan Lochte in 2011. Marchand beat Michael Phelps's long-standing 400m 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 1:52s is unbelievable for me." Marchand 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 this morning, clocking a time of 1:57.63s in the heats. He said he would go all-out for the record in the semi-finals rather than conserve his energy for tomorrow's final. 'Whirlwind' In today's finals, Tunisia's Ahmed Jaouadi won the 800m freestyle thanks to a perfectly timed attack. Jaouadi made his move midway through the race and eased away to touch the wall in 7:36.88s, ahead of German pair Sven Schwarz (7:39.96s) and 400m champion Lukas Maertens (7:40.19s). 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 200m freestyle in 1:53.48s. Second was China's Li Bingjie (1:54.52s) and third Claire Weinstein of the US (1:54.67s). 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 US celebrated the biggest title of his career with gold in the 200m butterfly in 1:51.87s. Italy's Simone Cerasuolo pipped Russian Kirill Prigoda to 50m breaststroke gold, touching home in 26.54s. 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 US 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 tomorrow when she goes in the 200m 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.