Halep withdraws from St Petersburg event

St Petersburg - Top seed Simona Halep pulled out of the St Petersburg 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-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.

"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 Russia's 19-year-old wildcard Natalia Vikhlyantseva advance to the semi-finals, where she will meet either defending champion Roberta Vinci of Italy or France's Kristina Mladenovic.