The Portugal international has scored a staggering 19 goals in 11 games in all competitions so far this season and is already the favourite to retain the Ballon d'Or prize for 2014.
Mendes - who claimed earlier this week that Ronaldo would retire at Madrid at the age of 40, despite rumours of a bid from Manchester United - believes his client will go down in history as the best sportsman the world has ever seen.
"In addition to being the best footballer in history, he is the greatest athlete of all time," he told AS. "You cannot compare him with anyone.
"He is the best player in the biggest sport in the world, in which there is more competition than in any other. For that reason, he is the best sportsman in the world."
When asked what Ronaldo is worth should any club attempt and audacious bid, Mendes replied: "The price that is stated in his release clause – €1bn."
Mendes also believes the Portugal captain will be able to break his own personal record of 60 goals in a single season provided he can stay fit in 2014-15.
"I'm sure that he will beat his personal record if he does not have any physical problems," he added. "For Cristiano, nothing is impossible. He is going to continue evolving and improving until he retires.
"Notice how Cristiano is a winger – he's not a striker – and his numbers are outrageous. He plays very far from goal and has to make significant efforts to arrive into the box. For that, he deserves even more credit for what he achieves."