It was worth the effort.

She left with a pair of gold medals and a couple of records, too.

Zhang started off Thursday with a dominating victory in the women's 200-metre butterfly, finishing more than a body length ahead of two Americans in an Olympic-record time.

"My coach told me I didn't have to think too much," Zhang said through an interpreter. "Just be myself. I really wanted this medal."

About an hour later, the 23-year-old returned to the deck to swim for China on the 4x200 freestyle relay.

It was a race she hadn't planned for. It was a race her teammates never expected to win.

After all, the Australians had double-gold medalist Ariarne Titmus taking the opening leg. The Americans countered with freestyling star Katie Ledecky handling their anchor. The final was expected to be a duel between those two powerhouse teams.

Instead, it was the Chinese touching first in a race where all three teams eclipsed the previous world record.

China's time of seven minutes 40.33 seconds is the one that will go into the record books, sending Zhang to the top of the medal podium at the Tokyo Aquatics Centre for the second time.

"I didn't know I was doing it until I'd finished the 200 butterfly and our coach told me, 'You're in the relay,"' she said. "I didn't even know how to swim the 200 free, although I have the training qualities and levels for the 200 distances."

Clearly, she's a quick study.

Zhang will go down as one of the breakthrough swimming stars at the Tokyo Games. She also won a silver in the 100 butterfly.

Not bad for someone who had never claimed a major international title before these Olympics.

Zhang's best showing in three appearances at the world championships is third. Her lone race at the 2016 Rio Games produced a sixth-place finish in the 200 fly.

Now, she's a double Olympic champion.

"We knew Zhang would be swimming in the relay, but the coach told us not to tell her," said Li Bingjie, who swam the anchor leg for China. "She was the last one to know."

"We didn't expect to win the gold," she said. "We just tried to finish third because Australia and the United States are very strong."

Australian Associated Press