Tokyo Olympics: Surprise golden double for China's 'butterfly queen' Zhang

Tokyo - Zhang has been in sizzling form this year, heading into the Tokyo Games ranked first in the world

China's Zhang Yufei was ready to relax after winning the Olympic 200m butterfly on Thursday, but a last-minute change of plan meant she ended the day with two gold medals rather than one.

On a hectic morning, the 23-year-old Zhang took the gold in the butterfly with an Olympic record, adding to the silver she claimed on Monday in the 100m fly.

She then backed up an hour later to join China's team in 4x200m freestyle relay, which produced a world-record effort to shock red-hot favourites Australia.

It wasn't supposed to be that way for Zhang, who intended to warm down after her individual final in preparation for her next scheduled event, 50m freestyle heats on Friday.

But Zhang's coach pitched her into the relay without warning in a calculated gamble that ultimately proved to be a tactical masterstroke.

"I didn't know I was doing it (the relay) until I'd finished the 200m butterfly and our coach told me, 'you're in'," she told reporters.

"I didn't even know how to swim the 200 free relay, although I have the training qualities and levels for the 200m distances.

"At the Chinese national championships I went very fast, so maybe that's why the coaches asked me to join the relay."

Teammate Li Bingjie said Zhang's effort in the butterfly inspired the Chinese swimmers to success in the relay.

"We were excited and it made us determined to do our best at the relay -- we were all in tears," she said.

The individual gold was a redemption for Zhang, who has been dubbed "The Butterfly Queen" by adoring Chinese media.

She finished a disappointing sixth in 200m fly at the Rio Olympics and failed to emerge from the heats at the 2019 world championships.

But Zhang has been in sizzling form this year, heading into the Tokyo Games ranked first in the world.

She lived up to expectations, with a scorching Olympic record of 2mins 03.86sec, the third fastest time in history and the best since 2012.

Regan Smith of the United States was well over a second behind at 2:05.30, followed by fellow American Hali Flickinger on 2:06.65.

Zhang exploded from the blocks to assert dominance at the first turn and was motoring at world record pace when the race reached halfway.

Flickinger, who took silver at the 2019 world championships, was Zhang's closest challenger going into the last length but ran out of steam and was pipped to silver by Smith at the end.

Flickinger said she was "nowhere close" to her best, while Smith was delighted to slash more than a second off her personal best.

"I can sometimes struggle a bit on the last 50m but I really fought hard and it paid off," she said.