Third seed Janko Tipsarevic suffered a surprise defeat in Wednesday's men's play in the Kremlin Cup, falling to Karen Khachanov in straight sets.
Tipsarevic appearing in the ATP 250 hard-court event for the seventh time, was one of three seeds to bow out in Moscow as he was beaten 6-4, 6-4 by Khachanov, a Russian wild card.
Tipsarevic, a winner of the event in 2011, was on the wrong end of two crucial breaks in both sets as he was ousted in 67 minutes.
Second seed Andreas Seppi kept his hopes of retaining his title alive.
Seppi eased past compatriot Paolo Lorenzi 6-3, 6-2 in his tournament opener. The pair matched each other game for game until the eighth, with Seppi then edging ahead before winning set point at the third attempt in the ninth.
The second set was slightly more straightforward as Seppi broke in the third, saved three break points in the fourth and broke again in the seventh to help book his place in the quarterfinals.
Argentina's Horacio Zeballos' tournament was ended in the second round after a 6-3, 6-4 defeat to Kazakhstan's Andrey Golubev.
The 26-year-old qualifier won 85 percent of points on his first service to dump the number six seed out in an hour and five minutes.
Eighth-seeded Frenchman Adrian Mannarino lost to Russia's Teymuriz Gabashvili 3-6, 6-4, 6-4 in a marathon encounter.
After battling from a set down to level, the wild card spurned eight break points in the third game of the third set before eventually edging ahead in the seventh and holding to take the match in two hours, 37 minutes.
Number five seed Denis Istomin of Uzbekistan avoided an upset in a 6-0, 2-6, 6-1 victory over wild card entrant Andrey Kuznetsov.
After winning the first seven games, the world number 47 was then broken twice as he lost the second set before surging into a three-game lead in the third and seeing out the match.
Ivo Karlovic was untroubled on his serve as he saw off American Denis Kudla 6-2. 6-3. Sergiy Stakhovsky was beaten by France's Edouard Roger-Vasselin 7-5, 7-5.
The Ukrainian beat seventh seed Joao Sousa in the first round but Roger-Vasselin proved too strong — with the world number 65 taking on Seppi in the last eight.