CoCo Vandeweghe knocked world number one Simona Halep out of the Stuttgart Grand Prix on Friday with a commanding 6-4, 6-1 win but admitted she wasn't even aware of the score for large parts of the match.
The Californian's 74-minute demolition of the Romanian top seed follows Maria Sharapova's loss in the first round and the injury retirements of second seed Garbine Muguruza and German crowd favourite Angelique Kerber on Thursday in the second round.
Vandeweghe, ranked number 16, admits that this is her least-favourite surface, but she has still reached the first clay semi-final of her WTA career.
She will on Saturday face French sixth seed Carolina Garcia, who constructed a third straight fightback over third-seeded Elina Svitolina 6-7 (4/7), 6-4, 6-2.
"At one point in the first set, I wasn't even sure of the score," Vandeweghe said. "Maybe that's the secret."
The American held her nerve to break early in the second set and added an insurance break for a 5-1 lead and closed out the upset on her second match point a game later when Halep went long with a return.
"Clay is still not my favourite surface, it's certainly not my number one," added the American.
But with a Porsche 718 Boxster going to the tournament winner along with prize money, Vandeweghe admits she has plenty of motivation to win a first title on the alien surface.
"I'm one step closer, maybe if I keep it up I can win a new toy to take home to California."
Halep lost to an American for the first time since the 2017 Australian Open when she was stunned by Shelby Rogers in the first round).
"It was like a flash and I don?t really know what happened during the match," the Romanian said.
"She served really well and it was really tough to break her. I lost the serve in the first set and then I went a little bit down.
"I couldn?t feel the rhythm, I couldn?t feel the game, I missed a little bit too much. She was very strong today."
Garcia has made a habit of rallying to beat Svitolina, duplicating comebacks she completed last autumn in Beijing and at the season-ending WTA finals in Singapore.
The two-and-a-quarter-hour struggle featured nine aces from Svitolina and 10 from Garcia, who earned her first win of the season over a top 10 player.
Saturday's second semi-final will be contested between Estonia's Anett Kontaveit and fifth-seeded Karolina Pliskova of the Czech Republic.
Kontaveit needed almost three hours to beat Russia's Anastasia Pavlyuchenkova 7-5, 6-7 (6/8), 6-4 while Pliskova came from break points down in the final game to beat Roland Garros champion Jelena Ostapenko, 5-7, 7-5, 6-4.
CoCo Vandeweghe found her footing on clay as she upset Simona Halep in Stuttgart
Marathon win: Karolina Pliskova needed three sets to beat Jelena Ostapenko and make the semi-finals