Dmitrov taken down by Harris in Madrid as Kvitova reaches quarters
Czech Republic's Petra Kvitova returns the ball to Russia's Veronika Kudermetova during their 2021 WTA Tour Madrid Open tennis tournament.
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.