Australia's Cameron McEvoy continued his fine form by landing a bronze medal in the men's 50m butterfly at the world championships in Doha.

McEvoy (23.08sec) finished behind Portuguese teenager Diogo Ribeiro, the surprise winner in 22.97, and American Michael Andrew.

The 50m butterfly is not an Olympic event, but 29-year-old McEvoy, who became the oldest Australian to win a swimming world championship in the freestyle last year in Japan, has still to defend his title in Doha.

Nicknamed "The Professor", the former maths and physics student has long been known for his cerebral approach to swimming.

But the butterfly medal was hard for him to get his head around, having barely trained in the stroke before the event.

"A couple of days before this competition were effectively the first few strokes I did of butterfly this whole preparation since Fukuoka," McEvoy said.

"I was 18th in Fukuoka and third today, so this is quite a big step in six months. So it is a big surprise and it is very nice."

McEvoy will have a few days to rest before he starts his 50m freestyle title defence in the heats on Friday.

He is already looking forward to a tilt for an individual Olympic gold medal at Paris 2024, having won only minor medals in relay events at Games past.

While other top swimmers have skipped Doha to focus on their Olympic preparations, McEvoy says the February championships are a blessing for a sprinter.

"I'm very excited for it - the field's stacked, a lot of depth there," he said.

"So having it as a real race, you couldn't ask for a better set-up in my personal perspective."

McEvoy's 20-year-old teammate Isaac Cooper put in a storming swim in the 50m butterfly but finished just outside the medals in 23.12, edged out as he was unable to sustain a bright start.

Like Cooper, fellow Australians Sam Williamson and Brianna Throssell were also forced to settle for fourth place in their respective events.

Williamson was fourth in the 100m breaststroke, behind gold medallist Nic Fink, of the US, who led from start to finish.

Fink beat Italy's Nicolo Martinenghi into second, with three-time Olympic champion Adam Peaty third. Peaty, who has won the event on three occasions, had missed the past two editions.

"It's definitely crazy getting the first worlds gold in the 100 at the age of 30," Fink said.

Throssell also finished fourth as Germany's Angelina Kohler won the women's 100m butterfly, with fellow Australian Alex Perkins sixth.

The 23-year-old Kohler burst into tears in the pool after winning in 56.28, ahead of American Claire Curzan and Louise Hansson of Sweden.

Kate Douglass - the only American individual gold medallist from last year's worlds in Japan who came to Doha - comfortably defended her 200m individual medley title in 2:07.05, ahead of Sydney Pickrem of Canada (2:08.56) and Yu Yiting of China (2:09.01).

With agencies