Second-seeded Grigor Dimitrov has overcome tough resistance from qualifier Adrian Ungur before winning in three tight sets for a place in the quarter-finals of the Istanbul Open.

Dimitrov, who reached the semi-finals in Istanbul last year, won 7-5 4-6 7-5 on Wednesday to set up a meeting with sixth seed Jiri Vesely, who rallied to beat Roberto Carballes Baena 6-7 (5-7) 6-2 6-2.

Fourth-seeded Federico Delbonis defeated Dudi Sela 6-3 2-6 6-4 to also make the quarters, while eighth-seeded Albert Ramos-Vinolas beat Karen Khachanov 4-6 6-1 6-0 to advance.

In the remaining first-round match on Wednesday, Illya Marchenko defeated Maximo Gonzalez 6-3 3-6 6-1.

Australian top seed Bernard Tomic, after an opening-round bye, will face Argentine Diego Schwartzman for a berth in the last eight.