Coco Vandeweghe claims Simona Halep scalp in Stuttgart

The Romanian faltered at the decisive moments, surrendering her serve at 5-4 in the opening set and failing to convert any of her four break point chances, as she missed out on a last-four meeting with either Elina Svitolina or Caroline Garcia.

Halep remains the favourite to win a first Grand Slam at the French Open, which starts in May.

She lost last year's final at Roland Garros to Latvia's Jelena Ostapenko – one of three defeats in Major finals.

Top seed Caroline Wozniacki was forced to retire from her Istanbul Cup quarter-final against Pauline Parmentier on Friday with an abdominal injury.

Wozniacki won the opening set before France's Parmentier fought back to level the contest at 4-6 6-3. Unable to continue, Australian Open champion Wozniacki conceded the match before the start of the decider.

Up next for 32-year-old Parmentier is Romania's Irina-Camelia Begu or Donna Vekic.

After being broken in the first game of the opening set, Wozniacki took a 4-1 lead before her opponent drew level. The Dane then switched gears to claim the opening set.

Seeking her maiden victory over Wozniacki, Parmentier raced ahead 3-0 in the second set. Wozniacki took a medical timeout to deal with her injury but struggled thereafter and called it quits after Parmentier won the second set.

Earlier, twice grand slam champion Svetlana Kuznetsova made 21 unforced errors in her 6-1 1-6 7-5 defeat by Slovenia's Polona Hercog.

Hercog, 27, will take on Maria Sakkari of Greece who beat Dutch qualifier Arantxa Rus 6-3 7-6(6).