Top-seeded Barbora Strycova saved two set points to beat Carina Witthoeft of Germany 6-2 7-6 (8-6) in the Biel Ladies Open second round.
The 18th-ranked Czech let slip a 5-3 lead in the second set, then twice faced set point against the 69th-ranked Witthoeft before forcing the tiebreaker.
Strycova's 17-year-old compatriot, Marketa Vondrousova, also advanced to a first career quarter-final at a tournament debuting on the WTA Tour.
Vondrousova, a left-hander ranked No. 233, eased past 60th-ranked Annika Beck of Germany 6-1 6-3.
Second-seeded Carla Suarez Navarro of Spain exited, losing to Camila Giorgi of Italy 6-2 6-7 (7-5) 6-2.
In Friday's quarter-finals, Strycova will play seventh-seeded Julia Goerges as the only seeds remaining in the draw.
