There are some greats with unbelievable resumes playing at a super high level for decades. Nadal is a super player but his career is rather short compared to some players like Bill Tilden. Tilden from 1912 to 1930 won 138 of 192 tournaments played, lost 28 finals with a 907-62 match record. That's a winning percentage of 93.6. Players couldn't play all the majors every year because travel by boat would take many weeks and they would get out of shape. Still Tilden won 10 classic majors and 14 total majors if you include Pro Majors.
Pancho Gonzalez played at a high level for decades and was number one for many years, from the late 1940's to the 1970's. Arguably the greatest serve and the best player ever.
You have Rosewall, Laver, Sampras, Federer, Kramer, Budge, Borg, Lendl, Vines, Connors. Rosewall, Laver, Borg and Lendl for example all won over 100 tournaments in their careers.
I think Nadal has a chance to be the greatest ever but we'll have to see.