Former world number one Dinara Safina was forced to abandon her quarter-final against top seed Victoria Azarenka at the Andalucia Tennis Championships on Friday because of a lower back injury.
Azarenka of Belarus, winner in Miami last week, was leading 5-1 when Russian Safina, who has slipped to 81 in the rankings, retired from the event in Marbella.
"I feel bad winning in this way, above all against an opponent like Dinara," Azarenka said at a news conference.
"She has been suffering with her injuries for quite a while and I hope she recovers soon."
Azarenka will meet Sara Errani in Saturday's semi-finals after the eighth-seeded Italian beat fourth seed Alexandra Dulgheru of Romania 6-2 6-2.
The other semi pairs second seed Svetlana Kuznetsova of Russia and Romanian qualifier Irina Begu.
Wild card Kuztnetsova thumped Spanish qualifier Lara Arruabarrena 6-1 6-2 and Begu surprised sixth-seeded Klara Zakopalova of the Czech Republic 6-3 6-3.