Simona Halep withdrew from the St. Petersburg Ladies Trophy on Friday because of a knee injury, the same issue that she blamed for her opening-round loss at the Australian Open.
The fourth-ranked Romanian had said she was pain-free after her win over Ana Konjuh this week.
Halep was the top-seeded player in St. Petersburg. Her withdrawal sends Russian wildcard entry Natalia Vikhlyantseva into the semi-finals, matching her best showing at a WTA tournament.
Vikhlyantseva, ranked 115th in the world, will next play Kristina Mladenovic, who beat defending champion Roberta Vinci 6-1 6-4.
Also, second-seeded Dominika Cibulkova advanced by beating fifth-seeded Elena Vesnina 6-3 6-3. The Slovak will next face either Svetlana Kuznetsova or Yulia Putintseva.