Reigning Wimbledon champion Petra Kvitova and French Open winner Li Na of China both fell to straight defeats in the pre-US Open WTA hardcourt tournament here on Thursday. Czech seventh seed Kvitova was beaten 6-1, 6-2 by Germany's Andrea Petkovic, the 11th seed, while Li, seeded sixth, went out 6-2, 6-4 to 10th seeded Samantha Stosur of Australia. Having recovered from a thigh muscle injury, the 21-year-old Kvitova was starting her overseas campaign in Toronto which had expected to be topped by the US Open at the end of August. Li, 29, made sporting history at the French Open in June when she became the first player from China to win a Grand Slam singles title.