An ankle injury forced Maria Sharapova to retire from her quarterfinal match against Petra Kvitova at the Toray Pan Pacific Open in Tokyo on Thursday.
The Russian second seed was 4-3 down in the first set to the Czech fifth seed when she pulled out.
"Maria went over on her left ankle while serving, and, after a brief medical time-out, decided that she could not carry on," a message on her website said.
Sharapova, who won the French Open this year, is world No. 2 and enjoying her best season since a shoulder injury saw her drop out of the top 100 in 2009.
Kvitova, the Wimbledon champion, walks over into a semifinal against Vera Zvonareva, the No. 4 seed who beat another Russian, Maria Kirilenko 6-3, 6-3 earlier Thursday.