Top seed Elina Svitolina retired at 4-6 3-4 down to Marie Bouzkova, who progressed to the quarter-finals of the Guangzhou Open at the expense of the world number three.
Svitolina became Bouzkova's third top-10 victim this season, following wins over Simona Halep and Sloane Stephens at the Rogers Cup.
"Of course it was a really tough match, even if [Svitolina] wasn't 100 per cent," Bouzkova said afterwards. "She was fighting really hard, as much as she could, as much as her body allowed her to.
"It was mentally a tricky match for me, because I knew in the end she's not feeling her best. I tried to stay there, stay aggressive, and not give her any easy points."
Zhang Shuai, a two-time winner in this tournament on home soil, overcame Andrea Petkovic 7-5 6-4 and will take on Switzerland's Viktorija Golubic, who stunned fifth seed Katerina Siniakova 4-6 7-5 6-0.
Sofia Kenin is the highest seed left in the draw following a 6-4 6-2 triumph over wildcard Katarina Zavatska. Kenin's next opponent will be Jasmine Paolini, the Italian qualifier who bested Zheng Saisai 7-5 3-6 7-5 in a marathon encounter.
Veteran 2011 US Open champion Samantha Stosur also went the distance and prevailed 6-4 1-6 7-6 (7-5) against Bernarda Pera.
Number two seed Ekaterina Alexandrova came from a set down to beat Kristyna Pliskova 3-6 6-3 6-4 and book a place in the quarter-finals of the Korea Open.
The contest rattled along at a considerable pace, lasting only one hour and 32 minutes, with the decisive moment coming when Alexandrova claimed the third break point on offer having reduced the Pliskova serve to 0-40, before going on to close out the decider.
The world number 39 will face in-form American Kristie Ahn, who was also taken the distance by Romania's Ana Bogdan.
After a nightmare first set, Ahn turned things around to prevail 0-6 6-4 7-6 (7-2) courtesy of a dominant third-set tie break.
Magda Linette continued her own impressive hard-court form by seeing off Anastasia Potapova 7-5 7-6 (7-4) and will take on Kirsten Flipkens – an authoritative 6-3 7-5 winner against seventh seed Margarita Gasparyan.
On a day of comeback wins, Wang Yafan overcame Ysaline Bonaventure 4-6 6-3 6-0.