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"The conditions were better for me than yesterday, as it wasn't as hot and humid," Zakopalova said. "I didn't play my best tennis but I was fighting for every point and I'm happy that I'm through."
Zakopalova is seeded first in singles for just the second time at a WTA event and first in doubles also for the second time.
"I like to play doubles because it's good practice for singles," Zakopalova said. "I'm playing with Zahlavova Strycova, who is a very good friend of mine and a very good doubles player. We are seeded first as a team for the first time, so we will try our best."
Also booking a quarterfinal berth was third-seeded Argentine Paula Ormaechea, who withstood a rain delay, a 5-3 first set deficit and a 5-1 second set deficit against Kiki Bertens, winning the last 10 points of her 75 75 victory.
"When I got here in the morning, it was already raining," Ormaechea said. "So I knew it could be a long match. Also, I had already lost to her in Toronto last year, so I knew she was a tough opponent. But I knew what I had to do, and I was just focused on that.
"In Argentina the weather is the same as here. Really hot and humid, so for me, this is normal. I also had a lot of support from the crowd, which made me really happy."
Ormaechea's quarterfinal opponent will be Italian qualifier Nastassja Burnett.