Olympic champion Zheng storms home to reach US Open last 16

Zheng Qinwen pulled off an epic upset, winning the last seven games to stun Madison Keys and keep her unlikely US Open challenge going on Saturday.

The Paris Olympics gold medallist from China triumphed 1-6, 7-6 (7/3), 7-5, defying the vocal pro-Keys crowd and further demonstrating that she is headed back to her best after a lengthy recovery from elbow surgery last year.

The former world number four's ranking had dropped so far that she had to go through qualifying to earn her place in the main draw.

It has all made for an emotional spell in New York for the 23-year-old, who felt she "forgot how to compete" during her enforced absence but can now feel her competitive fire burning.

"I have a lot of emotions when I'm playing the qualie matches," she said. "I was even crying after I won the third match reaching to the main the draw, and you know, main draw is just the beginning.

"For today, after coming back from 0-5, the first thing in that moment I thought, 'Please, I don't want to be beaten 6-0.'"

She got her game on the board, and saved a match point at 5-1, and she was off and running.

"Then I felt I have a lot of energy. I can just keep pushing, keep fighting," she said.

"At the end when I won the match, it's like, 'Wow, that's incredible.'

"Finally, one time the luck is on my side because all these three, four months I been suffering a lot."

With few matches under her belt this year, Zheng said it took her some time to get to grips with Keys's big forehand.

And when she held serve for 5-1 in the third she could immediately feel Keys's conviction wane.

"I see there is some maybe doubt or, I don't know, not as clean (hitting) as before," Zheng said.

"She started to make a lot of mistakes in that moment, and I start to feel like, 'OK, I need to be there. I need to keep fighting, don't give up, and let's see what's going to happen.'"

Keys said her struggles were something that has been building. After she earned her big lead in the third, she said, "I got really scared, like I couldn't swing ... I felt like I couldn't play tennis anymore."

For Zheng, it was a reminder never to let go of a match until the final point.

"Tennis sometimes feels like rollercoaster," she said. "So this gives me also lesson that in a tennis match you always have to be there.

"It doesn't matter if you are winning or losing. You have a match point or you don't have a match point, you just need to concentrate every single point as long as you can."