Elina Svitolina was stunned by a Maria Sakkari comeback in the Silicon Valley Classic quarter-finals, while Aryna Sabalenka moved through on Friday.
Sakkari upset top seed Svitolina 1-6 7-6 (7-3) 6-3 at the WTA Premier event, saving four match points in San Jose.
The Greek seventh seed was trailing 5-2 in the second set before rallying for an incredible victory.
Sakkari will next face Zheng Saisai, who upset 17-year-old fourth seed Amanda Anisimova 5-7 7-5 6-4.
Zheng converted nine of 11 break points, including three from three in the final set, to battle through in two hours, 52 minutes.
Sabalenka and Donna Vekic, the second and fifth seeds respectively, moved into a semi-final clash.
While Vekic was too strong for qualifier Kristie Ahn 7-5 6-0, Sabalenka edged sixth seed Carla Suarez Navarro 3-6 6-2 6-4.