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World number one Simona Halep suffered a surprise defeat at the Stuttgart Open as opponent Coco Vandeweghe reached her first WTA Tour semi-final on clay.

Romania's Halep, who will still retain her ranking, lost 6-4 6-1 to the 26-year-old American.

Vandeweghe lost to Halep in her only previous clay-court quarter-final, but was untroubled in a comfortable display on her least favoured surface.

The world number 16 will play France's Caroline Garcia in the last four.

Garcia, ranked seventh in the world, fought back from a set and 3-0 down to earn a 6-7 (4-7) 6-4 6-2 win over Ukrainian third seed Elina Svitolina.

Estonia's Anett Kontaveit beat Russia's Anastasia Pavlyuchenkova 7-5 6-7 (6-8) 6-4 to progress, while former world number one Karolina Pliskova overcame French Open champion Jelena Ostapenko 5-7 7-5 6-4.

Meanwhile, top seed Caroline Wozniacki was forced to retire from her Istanbul Cup quarter-final against France's Pauline Parmentier with an abdominal injury.

Parmentier, ranked 122nd in the world, levelled after Wozniacki won the opening set, only for the Australian Open champion to quit before the decider.