Dmitrov taken down by Harris in Madrid as Kvitova reaches quarters
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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."
Two-time quarter-finalist John Isner battled past Serbia's Miomir Kecmanovic 6-4 7-6 (6) while 11th seed Shapovalov defeated another Serbian Dusan Lajovic 6-1, 6-3.
In the women's draw, Petra Kvitova, the champion in 2011, 2015 and 2018, had to work hard to beat Russia's Veronika Kudermetova 6-3, 4-6, 6-4 and reach the quarter-finals.
Eighth seed Belinda Bencia is also through after Ons Jabeur retired trailing 7-6 (2) 4-3.