Third seed Nadal was pumped up for his return and easily overpowered world number 43 Federico Delbonis of Argentina 6-2 6-1 in the first round but the 14-time grand slam winner admitted he was not at his best.
"The wrist needs little bit more time to be 100 percent recovered," said Nadal, who withdrew from the French Open in May with a wrist injury and then missed Wimbledon. "If there is not Olympic Games here I would not be here competing."
Nadal, the 2008 gold medallist who only confirmed his Rio participation last week, was supported throughout the one-sided match by a rowdy Brazilian crowd.
The 30-year-old Spaniard, who missed a chance to defend his Olympic title with a knee injury, said the pain of missing the 2012 London Games drove him to return to competition sooner than expected.
"It was tough for me to not compete in London, it was one of the toughest decisions in my career. I was close to not being here," said Nadal, who was Spain's flag bearer during Friday's opening ceremony.
"The feeling holding the Spanish flag is something that will stay in my mind for the rest of my life. It was an unforgettable thing for me."
(Editing by Frank Pingue)
