Iga Swiatek's tennis collapsed in the final set as Emma Navarro stunned the top seed and 2023 China Open champion 6-4, 4-6, 6-0 in Beijing on Wednesday.
The 24-year-old world number 17 plays either fellow American Jessica Pegula or Ukrainian Marta Kostyuk in the quarter-finals.
"I definitely had some adjusting to do after our last couple times playing," said Navarro, who had lost both previous meetings to Swiatek.
"I wanted to come out and play calm and steady and focused tennis.
"I did that, so I'm happy with the way the match went tonight."
Six-time Grand Slam champion Swiatek saved four break points in the first game of the contest but was fighting trouble throughout the opening set, firing a slew of shots just long.
The world number two from Poland powered past the American with six aces in the second set, but folded in the decider and went out with a whimper.
"I'm feeling good, but there's always stuff to get better at," Navarro added.
"Lots to work on, but I'll try to enjoy this one and then it's back to work tomorrow."
Swiatek arrived in the Chinese capital fresh off a victory in Seoul, the 25th title of her career.
She cruised past wildcard Yuan Yue in her Beijing opener and advanced to the last 16 when Camila Osorio retired hurt.
Also in the women's draw, Britain's Sonay Kartal beat world number five Mirra Andreeva 7-5, 2-6, 7-5 in another surprise to clinch her first top-10 win.
Kartal, ranked 81st, next plays world number 27 Linda Noskova of the Czech Republic in what will be her first 1000-level quarter-final.