Petra Kvitova stayed on course to win a fourth Madrid open title by beating Russia's Veronika Kudermetova on Monday to reach the quarter-finals. Kvitova, who won the tournament in 2011, 2015 and 2018, was pushed hard by Kudermetova, who is one of the form players on the tour, having won her first WTA title in Charleston last month. But Kvitova used all her experience and some excellent serving to come through, a 6-3, 4-6, 6-4 win booking her place in the last eight, where the Czech will meet either world number one Ashleigh Barty or reigning French Open champion Iga Swiatek. "Definitely it was a tough one," said Kvitova afterwards. "It was about serving a lot. It was just the one break in each set. I'm really glad I managed most of it mentally. Veronika was really on fire. Since she won Charleston, she really plays great. She will be a big champion. "The whole match I was not relaxed, but calm I would say. That's how I managed to bring the win." Eighth seed Belinda Bencic is also through after Ons Jabeur retired with a thigh injury when trailing 7-6 (7/2) 4-3. In the men's draw, Bulgaria's Grigor Dimitrov fell in the first round after losing in three sets to South African Lloyd Harris. Harris, who beat world number four Dominic Thiem and Denis Shapovalov en route to the final in Dubai in March, pulled off another upset by ousting Dimitrov 6-3, 3-6, 7-6 (7/5). The world number 54 held off a late fightback from 13th seed Dimitrov, who saved three match points in the final set before breaking back to force a tie-break. "It was a big up and down match, I had a set and a break, then I was a break up in the third too," said Harris. "I got a little bit nervous at the end but I'm happy to close it out." In his first appearance at the Madrid tournament, Harris will now face Australia's 22-year-old Alex de Minaur in round two. Two-time quarter-finalist John Isner battled past Serbia's Miomir Kecmanovic 6-4 7-6 (8/6) while 11th seed Shapovalov defeated another Serbian, Dusan Lajovic, 6-1, 6-3. ta/lp