Summer McIntosh bagged her second title at the swimming world championships as Gretchen Walsh defied illness to win gold and 12-year-old Yu Zidi of China narrowly missed out on a medal.

The 18-year-old McIntosh romped home in the 400m freestyle on Sunday's opening night in Singapore and gave another demonstration of her huge talent a day later in the 200m individual medley.

She came home in 2 minutes 6.69 seconds, with Alex Walsh of the United States second (2:08.58) and Canada's Mary-Sophie Harvey third (2:09.15).

"Going into the race tonight my goal was to put my head on the wall first, so to get that done is good," said the Canadian phenomenon.

"I'm not super-happy with the time, but honestly, at a world championship, my goal is just to go as fast as I can."

McIntosh will also race in the 400m medley, 200m butterfly and 800m freestyle in Singapore.

She is on track to join Michael Phelps as the only swimmer to win five individual titles at a single world championships.

"Still happy with the gold and hoping to keep up my streak next time," she said.

Yu was fourth in 2:09.21 in her first world championships final, having been fastest off the blocks and in third place before fading a little.

The schoolgirl will also compete in Singapore in the 400m medley and 200m butterfly.

"She's obviously phenomenally talented at such a young age and I think it will be interesting to see how she takes this meet," silver medallist Walsh said of the Chinese prodigy.

A "fragile" Gretchen Walsh shook off a stomach bug to power to a dominant victory in the 100m butterfly.

The world record holder took gold in 54.73 seconds – the second-fastest time in history – ahead of Belgium's Roos Vanotterdijk (55.84) and Alexandria Perkins of Australia (56.33).

The United States team has been hit with a bout of acute gastroenteritis and Walsh said she had been laid low heading into the race.

"The last couple of days my body has been fragile," said the 22-year-old.

"I've needed to give myself grace and luckily I had the morning to recover and rest and I used that," she added.

"That helped me enormously going into tonight."

Walsh set the world record of 54.60 seconds in May.

She said she had to "reevaluate my expectations" for the world championships after her battle with illness but surprised herself with her performance.

"I'm over the moon," she said. "I'm really happy that when it mattered, I was able to do that and get my hands on the wall." (AFP)