EXCLUSIVE By Adrian Boullosa
The Argentine forward played a key role in Barcelona's successful 2014-15 campaign as he guided them to the treble with a string of sublime performances.
Although Ronaldo beat Messi to the Liga top goalscorer award - scoring 48 goals compared to the Barca star's 43 - Nadal feels that the 28-year-old stands alone as the game's greatest.
"Barcelona have a great team with one special player, Messi. He is able to unlock a game from out of nowhere," Nadal told Goal.
"Cristiano Ronaldo has been putting in spectacular performances, but there really is no comparison with Messi.
"He is on another level than all the other players. He is the undisputed best. No player has the ability to do what he does."
The former Spain international then went on to stress that Madrid will be Barcelona's main rival for silverware in 2015-16, although he has questioned the strength in depth of Rafael Benitez's side.
"Madrid and Barcelona are always challenging for any trophy. Real have a great squad, plus they have Ronaldo," he added.
"They started last season very strong, but injuries hurt them in the end. A great team like Madrid should have more strength in depth.
"When someone like Luka Modric gets injured, you should have an adequate replacement."