Defending champion Marin Cilic made hard work of his Kremlin Cup second-round clash with Denis Istomin before prevailing 7-6 (7-3) 3-6 6-3.

Having edged a first set tie-break, Cilic could not complete a straight-sets win in Moscow as Istomin broke twice to give the top seed plenty to worry about.

Cilic had failed to put any pressure on his opponent's serve in the opening two sets, but that finally changed at the start of the decider as he won four successive games.

That was sufficient to seal the win, a final break in the ninth game enough to book a quarter-final with Andrey Kuznetsov, who cruised past eighth seed Mikhail Kukushkin 6-1 7-5.

Kuznetsov broke every service game of his Kazakh opponent in the first set, before cruising through the second to seal his first quarter-final of the year.

Fourth seed Philipp Kohlschreiber was also given a scare in his match with qualifier Aslan Karatsev, but the German was able to find a way past the youngster and into a last-eight match with Robin Haase.

The seventh game of the deciding set proved to be the crucial moment as Karatsev battled to fend off two break points, but the third was too much and Kohlschreiber marched on to a 7-6 (7-3) 2-6 6-4 win.

Elsewhere in the draw, Lucas Pouille enjoyed a 6-1 7-6 (7-5) win over Daniel Gimeno-Traver and wildcard Evgeny Donskoy beat Ricardas Berankis 6-4 7-5.