PRAGUE (AP) Kristyna Pliskova upset second-seeded Petra Martic 6-1, 7-5 to advance to the quarterfinals of the Prague Open on Wednesday.

Pliskova, ranked 69th, hit nine aces and 28 winners on the way to her first win over her 14th-ranked opponent. It's the eighth clay-court quarterfinal for the hard-serving Czech since she was runner-up in Prague in 2017.

''I feel I'm playing better on clay each year,'' Pliskova said. ''Everybody was saying I cannot move and stuff, but I don't think it's like this. I have more time, so it's good for me that clay is not that fast, I can focus on my game.''

Pliskova next faces Ana Bogdan, who advanced after Lesia Tsurenko withdrew.

Third-seeded Elise Mertens defeated Camila Giorgi 6-4, 6-2 and will play either Tamara Zidansek or Eugenie Bouchard in the quarters.

In the last first-round match, Laura Siegemund knocked out Mayar Sherif 4-6, 6-1, 6-0.

The Prague Open is the second European tournament since the WTA restarted amid the coronavirus pandemic, with strict health protocols in place.