Canada's 15-year-old teenager Summer McIntosh offered a mature performance at the World Swimming Championships that belied her tender age, outclassing reigning Olympic champion Zhang Yufei to win the women's 200 meter butterfly on Wednesday.

Zhang got off to a flying start in the final, securing the lead at the 100m wall, but McIntosh managed to stage a comeback and ultimately bagged her first career world championship gold. Hali Flickinger also had a late rally and finished second, while Zhang grabbed bronze.

"I just literally gave it my all and did everything I could, and put in all my energy and all my focus, and just stretched for the wall and put my hand on the wall as fast as I possibly could," beamed McIntosh, who became the youngest individual world champion since 2011. 

Later in the day, the indefatigable McIntosh led Canada to bronze in the women's 4x200m freestyle relay with an opening leg that would have been won her the 200 free individually. The U.S. team edged defending champions Australia to reclaimed the world title.

The result means that veteran American star Katie Ledecky earned her 21st world championship medal, breaking her tie with compatriot Natalie Coughlin for the most by a woman in history.

The Chinese quartet of Tang Muhan, Li Bingjie, Ai Yanhan and Yang Junxuan combined to finish fourth, failing to replicate their free relay success in Tokyo where China broke the world record and claimed the Olympic gold.

In the other individual finals, Romanian 17-year-old David Popovici asserted himself as an emergent force after beating Frenchman Maxime Grousset to win the men's 100m freestyle.

It was his second gold after dominating the 200m free earlier this week. The latest triumph means Popovici became the first swimmer to sweep the 100m and 200m frees at the world championships since 1973.

"I'm glad we got to write a small page in swimming history," Popovic told reporters. "Some would say a huge page, but we like to keep it low key. I'm glad to have got two golds now, I think it will be pretty heavy carrying them."

When asked whether he had scared off reigning Olympic and world champion Caeleb Dressel, who had withdrawn on unspecified medical grounds, Popovici said, "I don't think so, I think he is too big of a boy to be running away from someone like myself or frankly anyone but I hope he's OK and I hope he'll come back stronger."

The teen boy then reminded the world of his young age by adding, "I haven't finished high school yet I don't even drive yet."