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The 20-year-old reigning French Open champion will next face Petra Martic of Croatia who beat Czech 26th seed Barbora Strycova 7-5, 6-4 on Friday.
Ostapenko, of Latvia, beat the top seed here Simona Halep in last year's French Open final in three sets, becoming the lowest ranked and first unseeded player in history to win at Roland Garros.