The Olympic champion battled her way through qualifiers amid a career slump and has recaptured her top form in New York, getting the best of an opponent who had led 7-to-1 in their head-to-head before Monday.
She appears to do her best work with her back against the wall, recovering from five games to love in the deciding set of her third-round match this week, and she found the magic again at Arthur Ashe Stadium, saving three set points to the increasing frustration of the 2022 champion Swiatek.
Swiatek took a medical timeout after the third game of the second set, having a physio come and massage her left thigh, but did not appear to struggle as she returned to the court.
Zheng broke her opponent with a backhand winner down the line in the seventh game and broke again on the fourth match point, leaving Swiatek in tears again two years after she stunningly beat the Pole in the Olympic semi-finals.
"I have to say I start the match not so well and after little by little I found my rhythm," said Zheng, whose chronic right elbow injury stunted her career and forced her to undergo surgery last year.
"I just focused point by point and little by little I started to play better tennis."
She next plays either Naomi Osaka or Elena Rybakina.