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Wimbledon champion Petra Kvitova became the fifth top 10 seed to exit the US Open in the first week when she was knocked out by Serbian world number 145 Aleksandra Krunic on Saturday. Third seeded Kvitova lost the third round tie 6-4, 6-4 to the pint-sized 21-year-old from Belgrade, who came through qualifying and had never won a match at a major before this year's event. Kvitova joins second seed Simona Halep, fourth-seeded Agnieszka Radwanska, Angelique Kerber, the sixth seed, and eighth-seeded Ana Ivanovic in failing to make the fourth round. The 24-year-old Kvitova dropped serve five times and committed 34 unforced errors against the 5ft 4in (1.63m) Krunic who divides her time between the pro tour and studying for a university economics degree.