Second-seeded Roberta Vinci of Italy advanced to the Istanbul Cup quarterfinals when Romanian qualifier Alexandra Dulgheru retired with a right shoulder injury while trailing 6-3, 4-1 on Thursday.
Vinci also beat Dulgheru last week in Bucharest en route to her first WTA final in a year.
Vinci next faces sixth-seeded Kurumi Nara of Japan, who beat Czech qualifier Katerina Siniakova 7-6 (5), 6-7 (3), 6-2.
No. 8-seeded Karolina Pliskova of the Czech Republic downed Shahar Peer of Israel 6-2, 7-5 to line up top-seeded Caroline Wozniacki in the quarters.
Also, Croatian qualifier Ana Konjuh reached her first quarterfinals, and became the youngest at 16 to make the last eight on the tour in almost two years, by outlasting Misaki Doi of Japan 6-4, 6-7 (3), 6-4. Konjuh meets rising star Elina Svitolina of Ukraine on Friday.