The retired 22-time Grand Slam champion is backing the world number one - who is set to make his court comeback on home soil at the Rome Masters in May following a three-month ban for testing positive for the banned anabolic steroid, Clostebol, in March 2024 - because he thinks Sinner "did not want to do something that is not allowed".
He told CNN: "First of all, I 100 per cent believe that Jannik is innocent. I don’t think at all that Jannik wanted to do something that is not allowed, so I 100 per cent believe in Jannik.
"Second, talking about the process, for me, I believe in justice. For some reason, we have the justice there that they have to make decisions. They have all the information that probably not one of us has and they make the decisions."
Nadal's comments come as the three-time Grand Slam winner agreed to a ban lasting from February 9 to May 4 after the World Anti-Doping Agency (WADA) lodged an appeal with the Court of Arbitration for Sport (CAS) when the International Tennis Integrity Agency (ITIA) ruled he was not to blame for the positive doping test because the contamination was caused by a physio applying an over-the-counter spray on him.
Despite players criticising the current anti-doping protocols for favouring top stars, including retired player Serena Williams' recent admission that she would be have been slapped with a "20-year" ban and had "Grand Slams taken away" from her - of which Sinner will not miss - Nadal "really believes" in the system.
The Spaniard said: "From my point of view, I really don’t believe that Jannik, because he’s the number one in the world, received different treatment than another person, from my perspective and from my understanding.
"I really believe in the process, I have been there going through all the tests for 20 years, how the things are strict on every single movement … and I believe in the process.
"I can’t say another thing and I can’t think another way because, if not, I will think that we are not in a fair world, and I really believe that we are in a fair world in this matter."
