Top seed and home favourite Angelique Kerber was sent crashing out of the Nurnberger Versicherungscup after a 7-6 (7-5) 6-4 defeat to Karolina Pliskova.
It was Pliskova's first victory over a top-10 player and even more impressive considering she won just one of the first 17 points.
"She was playing really amazingly in the beginning and there wasn't very much I could do. But then I won one game and suddenly I started to feel much better and I came back to win the first set," the Czech player told the WTA's official website.
"I sensed she was a little bit down at the start of the second and I made a really fast start.
"I then started to make a few mistakes that perhaps I shouldn't have and made it a little bit complicated. I'm happy that I got through in the end."
There was no such trouble for second seed Eugenie Biouchard, the Canadian progressing to the last four with a 7-6 (7-3) 7-6 (8-6) victory over Yaroslava Shvedova.
Seventh seed Caroline Garcia was also dumped out, the fast-rising Frenchwoman losing 6-7 (9-11) 6-4 6-4 to Italy's Karin Knapp, but fourth seed Elina Svitolina beat Mona Barthel 6-4 6-4 to advance.
In the semi-finals Pliskova will face Svitolina and Bouchard will go up against Knapp.