Spain's Feliciano Lopez upset eighth-seeded Argentine Juan Monaco 6-4, 7-5 Tuesday in the first round of the Serbia Open.
Lopez made the decisive break in the 11th game of the second set on a wind-swept day at the ATP tour event organized by the family of Novak Djokovic.
"To beat Monaco on clay, you have to be confident," Lopez said.
Sixth-seeded Ernests Gulbis also went out, losing 6-2, 6-4 to Illya Marchenko of Ukraine.
The 108th-ranked Ukrainian, who converted four of five break points, next faces the winner of the late match between Serbian wild card Dusan Lajovic and Filippo Volandri of Italy.
India's Somdev Devvarman rallied to beat Mikhail Kukushkin of Kazakhstan 1-6, 6-4, 6-2 and qualifier Adrian Ungur of Romania defeated Ruben Ramirez Hidalgo of Spain 6-7 (2), 6-3, 6-4.
Ungur will next play the second-ranked Djokovic, who is opening his clay court season on Wednesday after winning 24 consecutive matches this year, including victories at the Australian Open and hard-court tournaments in Dubai, Indian Wells and Miami.
Former No. 5 Fernando Gonzalez of Chile returns from an eight-month injury layoff when he plays Slovakia's Martin Klizan later Tuesday.