Eugenie Bouchard came through an enthralling encounter with Svetlana Kuznetsova to record a first win over a top-30 player in nearly two years.
The Canadian beat the number one seed 7-6 (7-3) 6-7 (5-7) 6-2 in a match just short of three hours at the Istanbul Open on Thursday.
Bouchard, who had lost all three previous meetings with Kuznetsova, almost let the first set slip from her grasp and squandered three match points in the second before recovering her poise to close out the win.
Speaking after reaching her third quarter-final of an encouraging year, the 26-year-old said: "That was extremely tough mentally.
"I blocked out mentally that I had missed match-point opportunities, so I had this anger – but I was calm. I had this calm anger at the start of the third set. That motivated me – I wanted to do the right thing after messing up. I wanted to make things better."
Bouchard will face Danka Kovinic next after the Montenegrin beat sixth-seed Alison Van Uytvanck 6-3 6-4.
Fourth seed Caroline Garcia is also out, falling 6-1 6-4 to Tereza Martincova, who will next meet Aliaksandra Sasnovich.
Third seed Polona Hercog will face Paula Badosa in her quarter-final after beating Jasmine Paolini in straight sets, while Patricia Tig and Rebecca Peterson will contest the other last-eight match.