Teenage phenomenon Summer McIntosh captured gold in ominous fashion on the opening day of the world swimming championships on Sunday as Australia claimed a relay double in Singapore. The 18-year-old Canadian launched her packed program with a dominant performance in the women's 400 meters freestyle, romping home in 3 minutes, 56.26 seconds, nearly two seconds clear of China's Li Bingjie, who was followed by American great Katie Ledecky in bronze. It was world record holder McIntosh's first world title in the event as she pursues five individual golds in Singapore. With Australia's Olympic champion Ariarne Titmus skipping the championships before she prepares for the 2028 Los Angeles Olympics, the race was billed as a showdown between McIntosh and Ledecky. But McIntosh grasped control early on and streaked further ahead as the race progressed. "I think the 400 freestyle in past world championships and Olympics, I haven't been at my best and I haven't been where I want to be," she said. "So to finally stand on the center of the podium is promising for the rest of the meet." McIntosh, who broke three world records at the Canadian trials in the lead-up to the worlds, jumped straight back into the pool for the 200 meters medley semifinals 20 minutes after winning gold. She qualified for the final with the fastest time, saying she was "in the best shape of my life". "Now I just have to act on that and put it into all of my races," she added. McIntosh was joined in Monday's 200 meters IM final by Chinese 12-year-old Yu Zidi. The schoolgirl, who juggles swimming with homework, squeezed into the final. "I'm in? Oh, I'm happy about that and I will continue to work harder," she said. "I hope to find a breakthrough at these world championships and show my potential." Germany's Lukas Maertens won a nail-biting gold in the men's 400 freestyle, pipping Australia's Sam Short by 0.02 seconds. Maertens, the Olympic champion and world record holder, became world champion in the event for the first time. German compatriot Florian Wellbrock won four golds in the open water events in Singapore and Maertens said their country had taken a "big step forward". "All the girls and guys are a really young team and there are a lot to come," he said. "Now I'm one of the old guys and I'm 23. That means a lot and I'm really proud to be in the German team." Maertens came home in 3 minutes, 42.35 seconds after a fierce battle with Short, the 2023 world champion. South Korea's Kim Woo-min, the reigning world champion, was third. Maertens was the favorite after breaking the world record in Stockholm in April, a mark that had stood since 2009. But Short fought him every stroke of the way, losing out after an incredible tussle to the finish. "There was a lot of pressure, also as the world record holder, it wasn't easy to fight these guys," said Maertens. Australia ended the opening night on a high after both their women's and men's teams won the 4x100 meters freestyle relays. The women's team of Mollie O'Callaghan, Meg Harris, Milla Jansen and Olivia Wunsch came home ahead of the United States, with the Netherlands third. Flynn Southam, Kai James Taylor, Maximillian Giuliani and Kyle Chalmers then completed the double, finishing ahead of Italy and the Americans in the men's race. Their winning time of 3:08.97 was a world championship record. "We've come together really well in camp and preparation -- we believed we could do it and we did it," said Chalmers. The U.S. team is battling a stomach bug in the camp, with two competitors skipping their opening races. Torri Huske, a treble gold medallist at last year's Paris Olympics, and 18-year-old Claire Weinstein did not compete "so they could focus on other events", team official Nikki Warner told AFP. "Our team experienced acute gastroenteritis," Warner said. "No one has pulled out of the competition." Huske later competed in the relay final.