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.

Former world number one Karolina Pliskova meets reigning French Open champion Jelena Ostapenko later on Friday, with Estonia’s Anett Kontaveit facing Russia’s Anastasia Pavlyuchenkova in the other quarter-final.

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