Georgian Nikoloz Basilashvili upended top-seeded Austrian Dominic Thiem, 6-4, 6-4, on Thursday to advance to the quarterfinals of the Koza Sofia Open in Sofia, Bulgaria.
Basilashvili saved all three break points against him to dispatch his foe in 1 hour, 18 minutes.
"I think I played really well on the important points and kept focus all the way through the match today," Basilashvili said. "This is very special and one of the best wins of my career, if not the best one. It will give me a lot of confidence for the upcoming matches."
The victory was Basilashvili's second career over a Top 10 player, with the first coming against Tomas Berdych in October.
Basilashvili advances to face eighth-seeded Slovak Martin Klizan, who overcame a tough first set to post a 7-6 (3), 6-2 victory over German qualifier Daniel Brands.
In other matches Thursday, fifth-seeded Gilles Muller recorded a 6-3, 7-5 triumph over Russian Mikhail Youzhny. Muller won the final three games of the match to set up a date with defending champion Spaniard Roberto Bautista Agut, who defeated the Luxembourg native in the Sofia semifinals last year.
Ninth-seeded Serbian Viktor Troicki, who was last season's tournament finalist, pulled out a 2-6, 6-4, 7-6 (3) win over Robin Haase of the Netherlands. He'll face third-seeded Bulgarian Grigor Dimitrov, who advanced following a 4-6, 6-3, 7-5 victory over Poland's Jerzy Janowicz.