Rafael Nadal admits he was shocked to learn of Andre Agassi's drugs revelations and is 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."
Wimbledon finalist Andy Roddick said Agassi remained his hero despite the revelations.
"Andre is and always will be my idol. I will judge him on how he has treated me and how he has changed the world for (the) better," Roddick wrote on his Twitter page.
Roddick said Agassi's letter to the ATP, in which the eight times grand slam champion duped the governing body into believing he had failed a drugs test because his drink had been spiked, came at a time when the player was far from his peak.
"To be fair, when Andre wrote the reported letter, he was well outside the top 100 and widely viewed as on the way out," said Roddick.
United States Fed Cup captain Mary Jo Fernandez said Agassi had been brave to come out with his secrets.
"It takes a lot of guts and courage to come out and say something that nobody would have really known about," Fernandez said.
Women's world number two Serena Williams reacted to the news by plugging her own book.
"I don't even know what crystal meth is so, you know, that's what my reaction to it is. I haven't read anything about Andre Agassi's book. All I know is that I have a book coming out," she told reporters at the WTA Championships in Doha.
Serena's sister Venus, the world number seven, added: "His book will probably sell. It seems very interesting, to say the least."