Top seed Caroline Wozniacki moved into the semi-finals of the Brussels Open when Yanina Wickmayer retired injured.
After losing the first two games, Wickmayer had treatment on her back and, at 40-0 down in the third game, pulled out at the Royal Primrose Club.
Wozniacki will now play French Open champion Francesca Schiavone, who beat Japan's Ayumi Morita 6-2 6-3.
Third-ranked Vera Zvonareva beat Alexandra Dulgheru of Romania 6-0 6-1 to set up a match against Peng Shuai.
Zvonareva - looking for her second title of the season - won the opening 10 games before seeing out a comfortable win.
Peng, who is ranked 31st, defeated Sofia Arvidsson to reach the last four, though she will have her work cut out ending a run of seven straight defeats to Zvonareva.
The tie of the semi-finals is no doubt the Wozniacki-Schiavone match, though. The Italian beat Dane Wozniacki the last time the two met on clay in the quarter-finals of the 2010 French Open.
"She is a great player, so it won't be easy," said Wozniacki, who has beaten her three times on hard courts since then.
And Wozniacki also had words of comfort for quarter-final opponent Wickmayer.
"To see her in so much pain right now, it is unfortunate," she said. "And especially in front of this amazing crowd."