Coco Vandeweghe defeated world number one Simona Halep in straight sets on Friday to reach the semi-finals of the Stuttgart Open.
The American won 6-4 6-1 in the WTA Premier event on clay at the Porsche Arena, and will face third seed Elina Svitolina or Caroline Garcia for a place in the final.
Halep, who defeated the world number 16 with ease in their only previous meeting at last year's Madrid Masters, may have been feeling the effect of her exertions in helping Romania return to the Fed Cup's World Group courtesy of a play-off victory over Switzerland last weekend.
She saved two breaks points in the sixth game as a tight first set remained on serve, but was eventually broken in the 10th game when a double fault allowed the composed Vandeweghe to deservedly move ahead in the match.
Vandeweghe made a determined start to the second set, holding before breaking Halep again with a powerful forehand winner off a weak second serve.
And the American showed great determination in the next game, recovering from 0-40 down and saving three break points to hold and take a 3-0 lead.
The top seed did manage to hold in the next game, but appeared increasingly unable to halt the momentum of her opponent, who saved another break point before controlling a rally with authority to move 4-1 ahead.
Halep was broken again as she came to the net and saw Vandeweghe fire off a dismissive cross court backhand winner to make it 5-1, before the American sealed victory with her second match point when Halep sent a forehand serve return long.