US swimmer Nic Fink claimed gold in the men's 100m breaststroke as British star Adam Peaty had to settle for bronze on his world championships return in Doha on Monday.

Fink, a silver and bronze medallist at the past two editions, won in 58.57 seconds, with Italy's 2022 world champion Nicolo Martinenghi taking silver, 0.27sec adrift.

Double Olympic champion and world record holder Peaty rounded out the podium, clocking 59.10sec.

The 29-year-old is appearing at the world championships for the first time since 2019. He took time out of the sport last year to concentrate on his mental health.

In Peaty's absence, China's Qin Haiyang swept the 50m, 100m and 200m breaststroke events in Fukuoka, Japan last year, emerging as a serious threat to Peaty at the Paris Olympics.

The 24-year-old Qin did not travel to Qatar for these world championships.

Peaty described his bronze medal performance as "bittersweet".

"The swimming speed is fine, over the next five months, I know that is only going to improve and that is my thing, but the skills are just not good enough at this moment," he commented.

"So it is bittersweet because we did not come here for medals, I wasn’t really to fuss about aiming for a medal here because that is just not the target, we’ve come off a hard bank of work and last night gave me a glimmer of hope thinking I could get faster even today."

Portuguese teenager Diogo Matos Ribeiro took the men's 50m butterfly gold by a tenth of a second from Michael Andrew of the USA and Australian Cameron McEvoy.

The 19-year-old is the world junior record holder and he was going one better than the silver he picked up at the worlds in Fukuoka last year.

He made history for Portugal in the process, claiming the country's maiden world swimming title.

Fink's fellow American Kate Douglass defended her 200m individual medley title in a time of 2min 7.05sec, less than a second outside the world record.

Canada's Sydney Pickrem took silver with China's Yu Yiting completing the podium.

The women's 100m butterfly title went to German Angelina Kohler in 56.28. She came in clear of American Claire Curzan and Swede Louise Hansson.

Kohler lined up in the final as the top qualifier and duly bagged her first world medal of any colour at her fourth championships.