Seeded players Fabio Fognini of Italy and Ryan Harrison of the United States were beaten in straight sets on Tuesday in the first round of the Serbia Open.
Benoit Paire of France beat the sixth-seeded Fognini 6-1, 6-4 and Joao Souza of Brazil defeated the seventh-seeded Harrison 6-3, 6-3.
Eighth-seeded Gilles Muller downed Lukas Lacko of Slovakia 6-0, 4-6, 6-3, and Spain's Guillermo Garcia-Lopez topped Igor Kunitsyn of Russia 7-6 (4), 6-7 (1), 6-2.
Victor Hanescu of Romania, Slovenia qualifier Aljaz Bedene, Antonio Veic of Croatia and Eduardo Schwank of Argentina all won.
The tournament is organized by the family of top-ranked Novak Djokovic.