Top seed Simona Halep was forced to withdraw from her St. Petersburg Ladies Trophy quarterfinal match in Russia on Friday due to a knee injury.
The 25-year-old Romanian said she had been suffering from tendinitis in the left knee since the BNP Paribas WTA Finals Singapore in October and it was the same issue after she lost at the Australian Open in the first round last month.
Halep, the world No. 4, knocked out Ana Konjuh in straight sets on Wednesday and had felt cautiously optimistic about the state of her knee, but began feeling pain after the match.
Halep, who was playing St. Petersburg for the first time, had been bidding for back-to-back wins for first time since making the semifinals at the 2016 Dongfeng Motor Wuhan Open, a run spanning four tournaments.
Halep's withdrawal sends wild-card entry Natalia Vikhlyantseva into the semifinals. The 19-year-old Russian, ranked No. 115 in the world, lost her only previous meeting with Halep 6-2, 6-2 at the 2015 Shenzhen Open.
Vikhlyantseva will next play France's Kristina Mladenovic, who beat defending champion Roberta Vinci of Italy 6-1, 6-4.
Mladenovic, the unseeded world No. 51, continued with the approach that saw her knock off American Venus Williams in the surprise of the second round.
"Roberta is a big fighter," the 23-year-old Mladenovic said after the match, "and she is very smart on the court. She tried different tactics and almost turned it around in the second set. I'm happy I stayed calm and all the time responded with a game plan as well."
Second-seeded Dominika Cibulkova of Slovakia advanced to the semifinals by beating No. 5 seed Elena Vesnina 6-3, 6-3 of Russia.
Cibulkova will next face Yulia Putintseva, who defeated fellow Russian and No. 3 seed Svetlana Kuznetsova 6-3, 6-7 (4), 7-5.