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 programme with a dominant performance in the women's 400m freestyle, romping home in 3min 56.26sec, 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 centre 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 200m medley semi-finals 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 200m 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 400m freestyle, pipping Australia's Sam Short by 0.02sec.

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."