Seeded players Andreas Seppi, David Nalbandian and Gilles Muller all won in straight sets on Wednesday to advance to the quarterfinals of the Serbia Open.
The second-seeded Seppi defeated Ivan Dodig of Croatia 6-4, 6-4, No. 4 Nalbandian beat qualifier Aljaz Bedene 6-4, 6-3 and No. 8 Muller downed Eduardo Schwank 6-3, 7-6 (0).
Joao Souza from Brazil also advanced after wild card Dusan Lajovic retired because of an ankle injury.
Top-seeded Pablo Andujar will face Filippo Volandri on Thursday.
Last year's champion, top-ranked Novak Djokovic, has pulled out of the tournament that is organized by his family.