Madison Keys of the United States stunned Wimbledon champion Marketa Vondrousova to reach the semi-finals of the US Open on Wednesday. Keys, a US Open finalist in 2017, produced a disciplined performance to clinch a 6-1, 6-4 victory in 1hr 26min on the Arthur Ashe Stadium. The American 17th seed will play second seed Aryna Sabalenka of Belarus on Thursday for a place in the final. Czech ninth seed Vondrousova, who made history in July after becoming the first unseeded woman to win Wimbledon, was left ruing an inability to convert any of nine break points against Keys.