Simona Halep has withdrawn from the St Petersburg Ladies Trophy before her quarter-final match with Natalia Vikhlyantseva due to a knee injury.
The world number four rebounded from her shock first-round exit at the Australian Open by winning her opening match in Russia 6-4 7-6 (7-2) against Ana Konjuh in round two on Wednesday.
However, the Romanian - who was seeded top in St Petersburg - will play no further part, meaning Vikhlyantseva, the world number 115, receives a walkover into the semi-finals.
Confirming the news on its official Twitter account, the WTA posted: "@Simona_Halep has withdrawn from @Formula_TX due to a knee injury."
Halep stated that pain in her left knee had troubled her on court when addressing her shock first-round exit in Melbourne to Shelby Rogers last month.