Barthel, chasing her third WTA title, will face Kaia Kanepi for the first time on Friday. The Estonian defeated another qualifier, Richel Hogenkamp of the Netherlands, 6-3, 6-1 to play in her third quarters this season.
In a tournament where none of the eight seeds made the last eight, there's another first-time clash: Alexandra Panova of Russia against Silvia Soler-Espinosa of Spain.
Panova, a one-time finalist on tour, beat Grace Min of the United States 6-2, 6-4. Soler-Espinosa, a first-time finalist in Strasbourg in May, got past Julia Goerges of Germany 4-6, 6-1, 6-3.