Former world No. 1 Caroline Wozniacki edged out reigning champion Dominika Cibulkova in three sets to reach the semifinals of the Kremlin Cup on Friday.
The quarterfinal in Moscow saw third seed Wozniacki win the first set with ease but struggle in the second at times on her way to a 6-2, 6-7(1), 6-1 win.
The Dane served for the match twice in the second, at 5-4 and 6-5 up, but Slovakia's Cibulkova broke her serve to love on both occasions.
The turning point came when Cibulkova needed treatment on her left ankle at 2-1 down in the decider, and did not win another game after that.
Wozniacki identified stamina as the key to victory.
"The difference was just that I could keep my highest form up until the end and win the key points."
She brushed off worries about her fragile service game. "I don't have any concerns about my serve. I had concerns about her return."
Cibulkova, meanwhile, told R-Sport she was keen to return to Moscow next year and recapture the title.
"Of course. Moscow, it's very special to me. It's my title that I will never forget," she said. Cibulkova beat Kaia Kanepi of Finland to claim last year's title.
Wozniacki's win stretches her lead to 9-4 in the pair's head-to-head stats, with all of her victories coming on hard courts.
The world No. 11 now faces either Sweden's Sofia Arvidsson or seventh seed Maria Kirilenko, the last Russian left in either the men's or women's draw in Moscow.