Saint Petersburg (AFP) - Top seed Simona Halep pulled out of the St. Petersburg Ladies Trophy on Friday because of a persistent knee injury, propelling a Russian wildcard into the semi-finals.

"I'm really disappointed that I had to withdraw from the quarter-finals today, but my knee is not okay," world No. 4 Halep 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. The Romanian crashed out of last month's Australian Open in the first round and again cited a sore knee.

"I've had a great time at St. Petersburg. The city and the tournament are beautiful and I look forward to coming back here and hopefully next time I will be better on court and also be healthy," Halep added.

Halep's withdrawal saw Russian wildcard Natalia Vikhlyantseva advance to the semi-finals, where she will meet either defending champion Roberta Vinci of Italy or France's Kristina Mladenovic.