Luxembourg fifth-seed Gilles Muller has secured a place in round two of the Sofia Open with a 6-3 6-2 victory over French qualifier Mathias Bourgue.
Martin Klizan, the eighth-seeded Slovakian, also came through in straight sets to beat Andrey Kuznetsov of Russia 6-0 7-6 (7-4) while Belgium's Steve Darcis defeated German qualifier Maximilian Marterer 6-3 7-6 (7-3).
Russian Mikhail Youzhny beat Serb lucky loser Marko Tepavac 7-6 (7-2) 6-4 to secure a second-round match against Muller, while Dutchman Robin Haase recorded a comfortable 6-4 6-3 win over wild card Dimitar Kuzmanov of Bulgaria.
German qualifier Cedrik-Marcel Stebe came from behind to beat Teymuraz Gabashvili of Russia 2-6 6-0 6-1 and will next play Spain's Roberto Bautista-Agut, the world No.16 given a bye into the second round.
