Nadal beat Agassi in their two meetings on tour
Rafael Nadal and Roger Federer have both said they are shocked by the news that tennis authorities covered up Andre Agassi's use of banned drugs.
Multiple Grand Slam winner Agassi said he lied about using crystal meth to the Association of Tennis Professionals (ATP) in 1997 to avoid a ban.
Nadal said: "If the ATP covered for Agassi then I think that's dreadful."
Federer added: "It was a shock when I heard. I am disappointed and I hope there are no more cases in future."
World number two Nadal, who played Agassi twice in his career, said the ATP's decision to take the American on his word without further investigation was a slight on his fellow tennis players.
I'm struggling to get my head around why Andre would want to confess to something so damaging as taking drugs and then getting away with it
"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," the Spaniard added.
Agassi, who won eight Grand Slams and is one of only six men to win all four major titles, said in his autobiography this week that he took the highly addictive drug with a former assistant and then lied to the ATP that his drink had been accidently spiked.
The ATP believed his story and took no action against the Las Vegan, who at the time had slumped to 141 in the world rankings despite having three Grand Slam titles already under his belt.
While world number one Federer was equally surprised, he admitted that Agassi, now 39, "has done a lot for tennis, both as a player and as a human being".
"Today, he raises millions of dollars for his foundation for disadvantaged children," Federer said.
And former US Open winner Andy Roddick insists his impression of Agassi will not be clouded by the drug use.
"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.
The American world number seven added: "To be fair, when Andre wrote the reported letter, he was well outside the top 100 and widely viewed as on the way out."
Yet Nadal said he could not understand the timing and why Agassi "now that he is retired, comes out and says this".
Do you take away a title he wouldn't have won if he had been suspended? He beat some people when he should have been suspended
He added: "It's a way of senselessly damaging the sport."
Boris Becker told The Times that Agassi's admission was "probably the most shocking thing I've heard in tennis".
The German, who played Agassi 14 times, added: "I'm struggling to get my head around why Andre would want to confess to something so damaging as taking drugs and then getting away with it. Why would he want to be so brutally honest?
"I'm really surprised that he would want to discuss such a private part of his life, to talk about such a bad period in his life. I'm sure this will help to sell his book. He doesn't need the money, though. He's a rich man."
Martina Navratilova also described the news as "shocking", adding: "Not as much shock that he did it as shock he lied about it and didn't own up to it."
She told the Associated Press: "Andre lied and got away with it. You can't correct that now. Do you take away a title he wouldn't have won if he had been suspended? He beat some people when he should have been suspended."
The ATP said it could not comment on Agassi's drug-taking revelation because it had withdrawn its doping case against him.
But the World Anti-Doping Agency (Wada), which has since helped tighten up drugs procedures in tennis, has asked the ATP to "shed light" on the case.
Dick Pound, the former head of Wada, said Agassi's admission should serve as a wake-up call to the ATP.
"It has got to the point where either these sports organisations enforce the rules or someone like congress is going to say 'we gave you a chance to manage your affairs properly and now we are going to take it over,'" said Pound, a senior Canadian member on the International Olympic Committee.
"The fact that one of the stars acknowledged that it is simple to beat the system tells you everything you need to know.
"He lied about the test but the most interesting part is the ATP and why they did not do anything."