Veronika Kudermetova in action during her quarter final match against Jessica Pegula on Wednesday. Reuters

The 26-year-old beat the American five-time Grand Slam quarter-finalist in three sets to reach a WTA 1000 event semi-final for the first time, recovering after a collapse in the second.

"To beat Jessica it feels very nice, I'm so proud of myself because every match I played almost three hours," said Kudermetova. "And today in the second set I start to play so badly. "But before the third set, I said 'Veronika, you need to try to play your best, because the fans are here. I want to show you some energy'."

The Russian battled well to take the first set in the Madrid sun, but crumbled in the second.

Kudermetova won three three-set matches to reach the quarter-finals and appeared to tire, making several unforced errors as last year's Madrid Open finalist triumphed easily in just 22 minutes, winning 83 percent of points.

However after a medical time-out she rallied in the third, with each player breaking the other twice for 3-3, before Kudermetova managed it again for a 4-3 lead which she was able to consolidate before serving it out.

Pegula saved one match point but Kudermetova clinched progress with her fourth ace of the match. The 12th seed will face world number one Iga Swiatek or Croatia's Petra Martic in Thursday's semi-final.

Agence France-Presse