Zheng Qinwen of China returns a shot to Elise Mertens of Belgium during the WTA 500 Charleston Open in Charleston, South Carolina, on Thursday. China's Olympic champion Zheng, seeded third, defeated 13th seed Mertens 6-3, 3-6, 6-3 to secure a spot in the quarterfinals, where she faces No 9 seed Ekaterina Alexandrova of Russia. "Today the weather was really humid and hot, I think it was really difficult to stay focused from the beginning to the end," Zheng said.