Grigor Dimitrov will meet Jiri Vesely in the quarter-finals of the Istanbul Open.

Second seed Dimitrov found the going tough against Romanian qualifier Adrian Ungur, ranked 165 places below him at 194th in the world.

The Bulgarian rallied from 3-1 down before going on to claim the first set, but he was broken to love late in the second as Ungur forced a decider.

After a tightly contested third set, Dimitrov claimed match point at the first attempt in a 7-5 4-6 7-5 triumph, setting up a date with sixth seed Vesely.

The Czech overcame world number one Novak Djokovic on clay at the Monte Carlo Masters this month and will be looking to avenge his second-round exit at the Bulgarian's hands in Bucharest in 2014.

Vesely also endured difficulties in booking his quarter-final berth, losing a lengthy first set against Roberto Carballes Baena before fighting back to win 6-7 (5-7) 6-2 6-2.

Federico Delbonis defeated Dudi Sela 6-3 2-6 6-4 and will meet Albert Ramos-Vinolas in the last eight.

After an early scare, Ramos-Vinolas cruised through sets two and three for a 4-6 6-1 6-0 victory against Karen Khachanov.

Illya Marchenko set up a second-round meeting with Damir Dzhumhur by beating lucky loser Maximo Gonzalez 6-3 3-6 6-1.