Rafael Nadal cruised to a two-set victory in his singles debut at the Brazil Open, but then the 11-time grand slam winner blasted the ATP Tour for its choice of balls.
The former world number one needed only 78 minutes to prevail over Brazilian Joao Souza, ranked 140th in the world, 6-3, 6-4.
Nadal advanced to Friday's quarter-finals where he will face Argentine Carlos Berlocq, who beat eighth seed Albert Ramos 7-6 (9/7), 6-4.
"It was an important victory for me," Nadal said. "the conditions are very difficult. The court is very fast, the ball too.
"The problem is not the court. The problem is the ball which is very bad," he added.
The 26-year-old Spanish star said the Brazilian organizers of the event were not to blame.
"It's the fault of the ATP for allowing play with a (tennis) ball of poor quality. The ATP does not have the capacity or the infrastructure to be able to analyze the conditions before the tournament begins," he added.
He said he made a "good decision" by withdrawing Wednesday from a scheduled doubles match he was to have played with Argentina's David Nalbandian against Argentine Horacio Zeballos and Austrian Oliver Marach.
Nadal said he was able to rest his knee, after a tough week of play in Chile last week, and made clear that his priority here was to play the singles event.
The $455,775 Brazil Open is part of the Latin American clay court circuit, along with the Vina del Mar and the Mexico Open in Acapulco where Nadal plans to play later this month.
He said he did not feel any particular discomfort during the match.
"I was more or less OK, perfect no. I am in a process of recovery," he noted. "The important thing is to have regularity, to be able to compete. I am confident that with the days and weeks, I will regain what I am missing."
Nadal, who reached both the singles and doubles finals Sunday at his comeback event in Vina del Mar, Chile, is the top seed at this week's event here.
The world number five, who has claimed seven of his 11 Grand Slam titles on the clay courts of Roland Garros, won the Brazil Open in 2005 when it was held in Costa do Sauipe in eastern Bahia state.
The tournament was moved to Sao Paulo last year when Spaniard Nicolas Almagro won.