Sergiy advanced into the Open BNP Paribas Banque de Bretagne Quimper challenger last eight following a 4-6, 7-5, 6-3 win over world no.163 Enrique López-Pérez on Thursday.
The Spaniard took the opening set with a break in the ninth game, but the third seed responded by breaking once in the second and third sets to clinch the victory in one hour and 58 minutes.
Stats: aces (16-9), double faults (3-2), 1st serve points won % (76-73), total points won (98-91).
Stako goes on to play against no.8 seed Peter Gojowczyk (GER) for a spot in the semis. The Kiev native and his doubles partner Jonathan Eysseric lost their doubles first round match to Purav Raja and Divi Sharan of India 7-6, 3-6, 9-11.