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Saint Petersburg -Top seed Simona Halep pulled out of the St Petersburg Trophy on Friday because of a persistent knee injury, as the seeds continued to crash at the tournament.
"I'm really disappointed that I had to withdraw from the quarter-final today, but my knee is not okay," the Romanian Halep, ranked fourth in the world, said.
"After my last match I felt pain and I feel it's risky to continue here."
Halep, who enjoyed an opening-round bye, beat Croatia's Ana Konjuh in the second round in St Petersburg.
The 25-year-old Halep crashed out of last month's Australian Open in the first round and again cited a sore knee.
Halep's withdrawal saw Russia's 19-year-old wildcard Natalia Vikhlyantseva advance to the semi-finals, where she will meet France's Kristina Mladenovic who knocked out defending champion and sixth seed Roberta Vinci of Italy 6-1, 6-4.
The Frenchwoman, who is seeking her maiden WTA singles title, knocked out Venus Williams in the second round on Thursday.
Mladenovic, ranked 51st, raced into a 4-0 lead before 33-year-old Vinci got on the board.
After taking the opening set in 26 minutes, Mladenovic produced one more break in the 11th game of the second set to advance to the semi-finals.
"I think it was a great performance again today," she said. "I tried just to copy my performance from yesterday. I started strong, served good, being aggressive."
Slovakia second seed Dominika Cibulkova, winner of last season's WTA finals in Singapore, also breezed into semi-finals with a straight-sets win over Russian fifth seeded Elena Vesnina 6-3, 6-3.
In Saturday's semis, Cibulkova will face unheralded Yulia Putintseva of Kazakhstan, who upset Russia's two-time Grand Slam winner and third seed Svetlana Kuznetsova 6-3, 6-7 (4/7), 7-5.
"Today I fought throughout the match and it paid off," Putintseva said.
"When I was 5-3 down in the deciding set I continued to play my tennis looking for the chances to win the match. Luckily everything ended well for me."
