Russian Ekaterina Alexandrova continued her underdog run through the Stuttgart Open by dumping out world No 3 Jessica Pegula in straight sets on Saturday.
Alexandrova, who knocked out rising star Mirra Andreeva on Thursday in the last 16, beat the American 6-0, 6-4.
She will play either world No 2 Iga Swiatek or Jelena Ostapenko on Sunday, with the final to be held on Monday.
Pegula reached the final at the Miami Open in March and won the Charleston Open earlier in April but did not have an answer against her opponent, who is ranked 22nd in the world.
After failing to win a game in the first set, the 31-year-old American saved three match points at 5-2 down in the second. She fought off two more at 5-4 but could not hold on.
Alexandrova's five WTA top-10 wins are the equal most in 2025 with Andreeva and American Madison Keys.
Later on Saturday, world No 1 Aryna Sabalenka faces Elise Mertens. A beaten finalist three times in a row from 2021 to 2023, Sabalenka has never won in Stuttgart.
In the other quarterfinals, two-time winner Swiatek, the world No 2, faces Ostapenko, while American world No 4 Coco Gauff takes on Italian Jasmine Paolini, who is ranked sixth in the world.