Olympic gold medallist Zheng Qinwen, who had to come through US Open qualifying, stunned six-time Grand Slam champion Iga Swiatek 7-5, 6-3 to reach the quarterfinals on Monday.
It was another gritty win for the Chinese star, a former world No 4 now ranked 121st in the wake of elbow surgery last year.
Zheng won the last seven games to stun Madison Keys in her previous match.
After falling behind 5-0 in the first set against Swiatek she again reeled off seven straight.
She broke the Polish star twice in the second set to clinch the win over the player she beat in the gold medal match in Paris two years ago.
"I guess to come back from 0-5 yesterday is not lucky, that's all I can say," Zheng said.
"I just focus on point by point," added Zheng, who saved three set points in the opening set.
Zheng, who said it took some time to adjust to the cavernous Arthur Ashe Stadium, made a number of sloppy errors as Swiatek raced to a 5-0 lead.
"Honestly, I have to say I started the match not so well,” Zheng said. “Little by little I found my rhythm on the centre court because it's been a long, long time. I just really appreciate it to be here in the Ashe Stadium.
”I'm just really happy to win this match because it's not easy, and it's been a while really I didn't beat any top 10 player."
Zheng was already the first qualifier to reach the US Open women's fourth round since Emma Raducanu – who went on to win the title in 2021.
Next she'll face either world No 2 Elena Rybakina or two-time US Open winner Naomi Osaka for a place in the semifinals.
Swiatek left the court in tears after another bitter Grand Slam disappointment.
She won her most recent major at Wimbledon last year but fell in the quarterfinals at the Australian Open in January before a fourth-round exit at Roland Garros and a third-round defeat in her Wimbledon title defense.