Nadal announced earlier that he will step back from tennis after the Davis Cup finals in November, ending a career which saw him win 22 Grand Slam titles.
"He's an unbelievable person. He taught us young players how to behave on the court, how to handle situations on the court... Also to stay humble at the same time, not changing with his success," said 23-year-old Sinner, speaking at the Shanghai Masters tournament.
"It's tough news for the tennis world and not only (the tennis world)."
Nadal is set to end his two decades as a professional with 92 titles and prize money alone of $135 million.
Despite his record-breaking career, Nadal was plagued by injuries, a painful by-product of his all-action, brutal-hitting style.
"Everything has a beginning... and also an ending," said Sinner. "Only he knows how he feels. It's a tough one."
It has been suggested that Sinner's rivalry with Spain's Carlos Alcaraz could be the new era version of Nadal's "Big Three" competition with Roger Federer and Novak Djokovic.
Djokovic will take on Czech teenager Jakub Mensik in the Shanghai quarterfinals on Friday.
"There are a lot of things we can take from them," said Sinner of the three veterans. "We cannot compare us with them. That's impossible, especially in this moment now."
"I think we all were very lucky to see the Big Three playing tennis, and I consider myself very lucky to get to know them also as a person and to learn from them."
