Top-ranked Caroline Wozniacki swept into the semifinals of the Brussels Open on Thursday when Yanina Wickmayer retired with an injured back after losing the first two games.
The Belgian needed a medical timeout after the second game to receive treatment on her lower back. Wickmayer came back, hit a few weak returns of serve and called it quits at Primerose club.
Wozniacki will play either French Open champion Francesca Schiavone or Ayumi Morita in Friday's semifinal.
Third-ranked Vera Zvonareva routed Alexandra Dulgheru of Romania 6-0, 6-1 to set up the other semifinal match against Peng Shuai.
Zvonareva won the opening 10 games before conceding one to Dulgheru. The Russian is looking for her second title of the season.
Peng, who is ranked 31st, defeated Sofia Arvidsson of Sweden 6-2, 6-4.