China's gold medalists Xu Jiayu, Qin Haiyang, Zhang Yufei and Cheng Yujie (R to L) pose with their medals after winning the mixed 4x100m medley relay. (Xinhua/Xu Chang)

Qin Haiyang claimed his second breaststroke title and guided China to their first mixed 4x100m medley relay victory, becoming the first swimmer to win three gold medals at the Fukuoka worlds.

Qin Haiyang earned his second breaststroke title and then guided China to their first mixed 4x100m medley relay victory at the World Aquatics Championships on Wednesday.

The 24-year-old added the men's 50m breaststroke gold medal to his 100m triumph two days ago.

America's Nic Fink, the defending champion, took silver and Qin's compatriot Sun Jiajun finished third.

After about an hour's rest, Qin joined teammates Xu Jiayu, the 2017 and 2019 backstroke champion, Zhang Yufei, the 200m butterfly winner, and youngster Cheng Yujie to seal the relay title, beating Australia and the United States into second and third places.

Qin, who swam the second leg, thus became the first swimmer to win three gold medals at the Fukuoka worlds.

"I feel so amazing," he said, adding that breaking the world record will be his next target.

But he acknowledged that he was very lucky to win the relay because he was very tired after the individual race.

"I was not in good form because of the 50m breaststroke final. Fortunately thanks to my teammates' excellent performance, we had the last laugh."

Zhang, the third leg, said, "This is our time, it's a great time. I have a beautiful and wonderful team."

France's Leon Marchand also completed a golden double with a victory in the men's 200m butterfly title, following his 400m individual medley victory on Monday.

In the women's 200m freestyle final, Mollie O'Callaghan broke the world record as she led Australia to a 1-2 finish.

The 19-year-old hit the wall first in a time of one minute 52.85 seconds, taking 0.13 seconds off the previous world record of 1:52.98, set by Italy's Federica Pellegrini.

Ariarne Titmus, the 400m freestyle winner, came in second with 0.16 seconds behind. Canada's Summer McIntosh, 16, finished third.

The men's 800m freestyle title went to Tunisia's Ahmed Hafnaoui, the Tokyo Olympic 400m gold winner.