Jessica Pegula got the better of Karolina Pliskova once again as she knocked the fourth seed out of the bett1open on Thursday.
The American beat the former world number one 7-5 6-2 in Berlin, setting up a quarter-final clash with Victoria Azarenka.
It is the fourth time this season Pegula has defeated Pliskova.
Azarenka was also a straight-sets winner, overcoming Angelique Kerber in a battling 6-3 7-5 victory.
The two-time Australian Open champion was 4-1 down in the second set but roared back to prevail over Kerber for the 10th time in 11 matches.
Garbine Muguruza enjoyed a 6-4 6-3 victory over Elena Rybakina and will take on Alize Cornet next, while Ludmilla Samsonova downed Veronika Kudermetova by the same scoreline and will now face Madison Keys.