Defending champion Pablo Cuevas made a shock first-round exit at the Rio Open as a trio of qualifiers and a wildcard won through.

Cuevas, the Uruguayan third seed and last year's winner, was edged by Belgian qualifier Arthur De Greef 6-3 3-6 7-5 on Monday.

De Greef, ranked 138th in the world, grabbed the decisive break of the third set in the 12th game.

The loss at the ATP 500 tournament played on clay in Brazil marked Cuevas' fourth straight defeat, having reached the quarter-finals in Sydney in January.

It was a good day for qualifiers and a wildcard in Rio.

Spaniard Roberto Carballes Baena needed just 66 minutes to breeze past Portuguese eighth seed Joao Sousa 6-3 6-1.

Argentina's Nicolas Kicker overcame Italian Marco Cecchinato 7-5 6-0 in a battle between two qualifiers.

Casper Ruud, an 18-year-old wildcard from Norway, stunned local hope Rogerio Dutra Silva 6-3 6-4.

Diego Schwartzman came through his opener, edging Renzo Olivo 6-2 3-6 6-4 in an all-Argentine clash.

Facundo Bagnis, another Argentinian, went down to Dusan Lajovic 6-4 6-4.