STUTTGART, Germany (AFP) - Unfancied American CoCo Vandeweghe knocked another big name out of the Stuttgart Grand Prix on Friday (April 27), as she stunned top seed Simona Halep 6-4, 6-1.

The 74-minute demolition of the Romanian in 74 minutes 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 No. 16, admits that this is the surface she likes least, but she has reached the first clay semifinal of her WTA career. She will face either French sixth seed Carolina Garcia or tournament number three Elina Svitolina on Saturday.

Vandeweghe won the opening set as Halep double-faulted for a third time.

"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 from a Halep return long.

"I just tried to keep the balls in play and stay in the rallies. I tried to do the best I could.

"Clay is still not my favourite surface, it's certainly not my number one," she said.

But with a Porsche 718 Boxster going to the tournament winner along with prize money, Vandeweghe admits she has plenty of motivation to try for a 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 (Shelby Rogers in the first round).