World number five Rafael Nadal has blamed the ATP for exacerbating the tricky playing conditions at the Brasil Open.

Many players and coaches have complained about the state of the indoor clay courts in Sao Paulo, but the Spaniard - speaking after his 6-4 6-4 win over Joao Sousa - believes that the balls are causing all the problems.

"The balls are very fast and with very little hair, it's hard to control," Nadal told reporters. "The problem isn't the courts, it's the ball.

"It's the ATP's fault, not the tournament's. The ATP doesn't have the capacity and infrastructure to choose balls before the start of the tournament."

Nadal faces Carlos Berlocq in the quarter-finals later today.