Top seed Sara Errani will face Mona Barthel in the final of the WTA Open GDF Suez in Paris on Sunday.
Errani, the world number seven from Italy, was leading Kiki Bertens 5-0 in the first set when the Dutch player retired due to back pains.
"I was in a lot of pain this morning even after hours of treatment," Bertens, from the Netherlands, explained.
"I'm going to rest now for three, four or five days and then we'll see how things go."
Germany's Barthel was a convincing winner in the second semi-final, seeing off Frenchwoman Kristina Mladenovic 6-1 6-4 in just 65 minutes.