China's Olympic champion Zheng Qinwen defeated 13th seed Elise Mertens of Belgium 6-3, 3-6, 6-3 to secure a spot in the quarterfinals at the WTA 500 Charleston Open on Thursday, extending her clay-court winning streak to 13 matches across seasons.

The third seed next faces ninth-seeded Russian Ekaterina Alexandrova with a 0-2 head-to-head record.

"Today the weather was really humid and hot, I think it is really difficult to stay focused from the beginning to the end," Zheng said. "I'm really happy to go through this match, and I want to say thanks to all the crowd who came to support me today."