It was No. 1-ranked Nadal's first match since losing three weeks ago in the Australia Open final against Stanislas Wawrinka, a match the Spaniard was a heavy favourite to win.
Nadal tweaked his back warming up for the final in Australia, bringing up questions again about his health to start a season. A year ago he was coming back from a left knee injury, which he overcame to win the French and US Open.
"Daniel is a specialist on clay, so for me it was a great match after so much time off after the final in Australia because of the back problems," Nadal said. "I practiced very little. So I was happy to get back, and get back with a victory."
Nadal looked fine on the outdoor red clay and seems headed for a possible final Sunday against fellow Spaniard David Ferrer.
Ferrer defeated Jeremy Chardy in his first-round match 6-2, 6-3.
Nadal broke Gimeno-Traver in the sixth game en route to taking the first set. The two were even in the second set until Nadal broke in the 11th game and then served out to win.
Nadal has the best clay-court record in the Open Era, 294-21.
"For Rio, for the Brazilian people, for everybody it's nice because he's No. 1 in the world," third-seeded Fabio Fognini said of Nadal. "For the tournament, especially because it's the first year, I think it will help."
Other seeded players advancing on Tuesday included: No. 3 Fognini, No. 4 Tommy Robredo, No. 7 Juan Monaco and 8 Pablo Andujar. Ousted were No. 5 Nicolas Almagro and No. 6 Marcel Granollers.
In the combined WTA event, top-seeded Klara Zakopalova of the Czech Republic won her first-round match, beating Colombia's Mariana Duque-Marino 6-4, 6-4. Second-seeded Francesca Schiavone of Italy was eliminated in a 6-4, 6-4 to Spain's Lourdes Dominguez Lino.
Unseeded Teliana Pereira defeated No. 4 Alexandra Cadantu 6-3, 6-4 to become the first Brazilian woman ranked in the top 100 in 23 years. The last was Andrea Vieira.
Brazil's most famous women's player is Maria Bueno, who won seven Grand Slams, the last at the US Open in 1966.
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