Rafael Nadal admits he was shocked to learn of Andre Agassi's drugs revelations and concerned that the American avoided punishment.
Agassi reveals in his new autobiography that he used crystal meth in 1997, the worst year of his career, and lied to the Association of Tennis Professionals when he was asked to explain a failed drugs test.
The tennis authorities have come under fire since Agassi's disclosure, with the worry that he was cleared because his explanation was taken as fact.
World number two Nadal said: "If the ATP covered for Agassi then I think that's dreadful.
"If they covered for the player and punished others for doing the same kind of thing then that would seem to me to be a lack of respect for all sportsmen."
Roger Federer, the world number one, was also stunned by Agassi's story.
Federer said: "It was a shock when I heard. I am disappointed and I hope there are no more cases in future."