Top seed Dominic Thiem was dumped out of the Garanti Koza Sofia Open while home favourite Grigor Dimitrov made a winning start in the Bulgarian capital.
Austria's Thiem, the world number eight, fell 6-4 6-4 to Georgian player Nikoloz Basilashvili in the second round while third seed Dimitrov came from behind to beat Jerzy Janowicz 4-6 6-3 7-5.
Dimitrov's victory marked a successful homecoming following his run to the semi-finals of the Australian Open last month and he was quick to acknowledge the support he received.
"The crowd won the match today," the world number 13 told atpworldtour.com. "I had tremendous support and everything was amazing.
"I haven't played at home in a long time, so I'm pumped for each match here and hope I can repeat this tomorrow."
Dimitrov, who made the crucial break at 5-5 in the deciding set, will play ninth seed Victor Troicki in the quarter-finals while Basilashvili will take on Martin Klizan, the eighth seed.
Troicki edged out Robin Haase 2-6 6-4 7-6 (7/3) while Klizan saw off qualifier Daniel Brands 7-6 (7/3) 6-2.
Fifth seed Gilles Muller also went through, beating Mikhail Youzhny 6-3 7-5.
Britain's Aljaz Bedene, the world 109, was beaten in the second round of the Open Sud de France by German teenager Alexander Zverev.
Bedene produced a battling display but eventually went down 7-5 3-6 6-4 in two hours.
Dustin Brown upset top seed Marin Cilic 6-4 6-4 to set up a clash with Benoit Paire.
Defending champion Richard Gasquet reached the last eight with a 7-5 6-2 defeat of Malek Jaziri while fellow Frenchman and second seed Jo-Wilfried Tsonga beat Pierre-Hugues Herbert 6-1 6-2.