Second seed Andy Roddick overcame the stubborn challenge of Brazil's Flavio Saretta to reach round two of the Olympic men's singles tennis event.
The American was kept on court for one hour and 22 minutes by Saretta, ranked 81st in the world, but finally prevailed 6-3 7-6 (7/4).
Eleventh-ranked Juan Ignacio Chela was the first seed to fall, losing in three sets to Max Mirnyi of Belarus.
But fifth seed Juan Carlos Ferrero and 13th seed Marat Safin went through.
Spaniard Ferrero beat Hicham Arazi 6-3 6-1 while Safin was a 6-0 6-4 winner over Karol Kucera of Slovakia.
The biggest upset of the day was fourth seed Tim Henman going out in straight sets to Jiri Novak of the Czech Republic.
Roddick, who fired down 12 aces against Saretta, next plays 2000 Olympic silver medallist Tommy Haas after the German beat Croatia's Mario Ancic 6-1 7-5.
And the US Open champion was unconcerned by the sparse crowd.
"People see tennis a lot. It was perhaps a choice between tennis and swimming but I wasn't too concerned," he said.
"I was just concentrating on wining the match."
Fourteenth seed German Nicolas Kiefer beat Belarussian Vladimir Voltchkov 6-2 6-4.
Sixteenth seed Fernando Gonzalez of Chile disappointed the Athens crowd by beating Greek player Konstantinos Economidis 7-6 (8/6) 6-2.
Sixth seed David Nalbandian of Argentina has withdrawn from the tournament with a left thigh strain. He will be replaced by Canadian Fred Niemeyer.