Top seed Elina Svitolina bowed out of the Ostrava Open WTA tournament after her first match as she lost to Maria Sakkari in straight sets on Wednesday.
Sakkari, the world number 23, saw off world number five Svitolina 6-3, 6-3 in an hour and 16 minutes to clinch a quarter-final spot.
"I think my groundstrokes worked really well, I was super solid from the baseline," the Greek told Czech Television after the match.
"And of course my serve helped me a lot," added Sakkari.
Jelena Ostapenko pulled off another surprise against fifth seed Petra Martic with a convincing 6-3, 6-1 win.
Fourth seed Victoria Azarenka beat Barbora Krejcikova 2-6, 6-2, 6-1 and Elise Mertens, seeded seven, eased past Amanda Anisimova 6-2, 6-3.
"My fighting spirit brought me into the game," said the 14th-ranked Azarenka.
"I started to pick up my game from there. I think it was the jumpstart -- my fighting spirit."