The fourth seed was far too good for Switzerland's Stefanie Voegele, sealing a 6-3 6-2 win on the clay at the WTA International event.
Voegele struggled to compete on serve, winning just 18 of 47 points, and German Petkovic had 13 chances to break as a result.
She took eight of them, meaning that even though she was broken four times herself, Petkovic was still a convincing winner.
Petkovic won the title in 2009 and will now play American Grace Min in her semi-final.
Min surprised eighth seed Karolina Pliskova of the Czech Republic 6-2 6-1.
World number 14 Sara Errani also progressed but had to come from behind to defeat South Africa's Chanelle Scheepers 2-6 6-1 6-3.
Second seed Errani won 81 per cent of points on her first serve and made six breaks.
Her next challenge will come against American qualifier Shelby Rogers, who toppled Italian Camila Giorgi 6-1 7-5.
Giorgi battled on serve and made 11 double-faults, allowing Rogers to claim five breaks and victory.