|File photo of Switzerland's Stan Wawrinka. (Source: REUTERS/Toru Hanai)|
The 32-year-old Wawrinka, who missed six months of the 2017 season to undergo two knee surgeries after Wimbledon, will face Viktor Troicki of Serbia on Friday for a place in the semi-finals.
"I expected a tough match," world number 15 Wawrinka told atpworldtour.com. "He's already played a few matches here."
Wawrinka had won just one match this year before the Bulgaria tournament, seeing off Ricardas Berankis at the Australian Open in January before falling to Tennys Sandgren of the United States.
Stan Wawrinka (SUI) bt Martin Klizan (SVK) 4-6, 6-2, 6-3
Viktor Troicki (SRB) bt Denis Istomin (UZB) 7-6 (7/4), 1-6, 7-6 (7/5)
Maximilian Marterer (GER) bt Joao Sousa (POR) 7-5, 7-6 (8/6)
Marius Copil (ROM) bt Blaz Kavcic (SLO) 6-2, 6-2
Gilles Muller (LUX) bt Andreas Seppi (ITA) 4-6, 7-6 (7/5), 7-6 (7/2)
Marcos Baghdatis (CYP) bt Adrian Mannarino (FRA) 6-7 (4/7), 6-3, 6-1