Barthel, seeded third and seeking her second WTA Tour victory of the year and her career, recorded a 6-3, 6-4 quarterfinal victory over Barbora Zahlavova Strycova.
That set up a meeting with surprise semifinalist Kirsten Flipkens, who advanced when Anna Tatishvili retired in the second set. Flipkens had a 6-2, 3-0 lead when Tatishvili called an end to the match.
Flipkens defeated top-seeded Dominika Cibulkova in the second round.
The other semifinal will send Kristina Mladenovic against eighth-seeded Lucie Hradecka.
Mladenovic breezed past Melanie Oudin 6-0, 6-1 while Hradecka was a 6-4, 6-2 winner over Lauren Davis.