Chinese 12-year-old Yu reaches 200m individual medley semifinals

China's 12-year-old swimming sensation Yu Zidi will join Olympic champion Summer McIntosh in the semifinals of the women's 200 metres individual medley after qualifying in the heats on her world championships debut in Singapore on Sunday.

There was no stage fright for the schoolgirl from China's northern province of Hebei as she posted a time of two minutes and 11.90 seconds, a second off her personal best, to finish 15th in the heats.

Yu made the swimming world sit up and take notice at national championships in Shenzhen in May by winning the 400 IM and 200 butterfly titles, as well as coming second in the 200 IM, all with qualifying times for Singapore.

Faster than Canadian McIntosh at the same age, Yu's potential is enormous and her ability to compete with the world's best will be closely watched over the coming days.

World record holder McIntosh is the overwhelming favourite to win the 200 IM, though she qualified second in the heats in 2:09.46 behind Australia's Tara Kinder.

Sunday's evening session will feature a showdown between McIntosh and American great Katie Ledecky for the 400 freestyle gold.

Twelve years after winning the first of her four 400 world titles at Barcelona, Ledecky qualified fastest for the final with a time of 4:01.04.

McIntosh, the Olympic silver medallist last year, will go for her maiden 400 title after finishing fourth in the heats.

The 18-year-old coasted to a time of 4:03.11, nearly 10 seconds adrift of her world record of 3:54.18 set at the recent Canadian trials.

Erika Fairweather, the reigning 400 world champion, was a shock disqualification for a false start after a small movement on the block.

She was one of several disappointments on day one's first session.

With food poisoning having hit the United States team during their pre-meet camp in Thailand, Olympic champion Torri Huske failed to take her place in the women's 100 butterfly.

Australia's Olympic silver medallist and former world champion Elijah Winnington, meanwhile, failed to qualify for the men's 400 freestyle final.

It was compatriot and former world champion Sam Short taking top seed for the final instead.

He will battle the German world record holder and Olympic champion Lukas Maertens for the gold later on Sunday.

Olympic champions Australia and the United States will battle for the women's 4x100 freestyle relay gold in the evening session, while a new-look US team topped qualifying for the men's 100 relay final.

Australia and China are also expected to battle for the gold in the men's relay later on Sunday.