Novak Djokovic eased to his 25th consecutive win of 2011 by beating Romanian qualifier Adrian Ungur 6-2 6-3 in the second round of the Serbia Open.
The world number two was making his first clay-court appearance of the season in his home city, Belgrade.
After being broken twice by Ungur, Djokovic said: "It's tough to play on clay after almost a year. It will take a bit to get used to it."
He now faces Slovenia's Blaz Kavcic in the quarter-finals.
Djokovic's last match was his victory over Rafael Nadal in the final of the Miami Masters on 4 April, but he looked some way short of that form.
In drizzly condition he dropped his serve twice and landed only 59% of his first serves.
But, even allowing for some rustiness, the Serb still had too much for Ungur, who is 173 places below him in the world rankings.
After allowing his opponent to recover from 3-0 to 3-2 in the opening set, the Australian Open champion secured six of the next seven games.
Djokovic then began to produce some of his best shots, with some sweetly-struck winners in the closing stages.
Spanish duo Albert Montanes and Feliciano Lopez joined Djokovic in the quarter-finals with wins over Germany's Michael Berrer and Fernando Gonzalez of Chile respectively.