@shawnbm said it well, but I will add a few things.
@calitennis127 , it seems that you're essentially saying that in a neutral context--one without any pressure, like a practice session--David Nalbandian is actually a technically superior player to Roger Federer. That the difference in their accomplishments mainly boils down to Roger's ability to compete. Yes? You might agree with some of shawnbm's refinements, but you seem to generally believe that Nalbandian's technically ability is superior and, presumably, as good as the tennis world has ever seen?
This is actually somewhat of a non-falsifiable statement. In other words, it is hard to prove you wrong because there is no real way to know, unless we had a panel of knowledgable judges watch each perform in practice sessions many times.
I am also reminded of the Harlem Globetrotters, who can do all manner of things with a basketball but would get slaughtered by any professional--or college--team. Nalbandian isn't quite the Harlem Globetrotters, but the analogy serves insofar as it points out that a huge aspect of performance is the ability to compete - to translate those technical skills to a competitive match.
I would also argue that a major part of Roger's greatness is that he can do pretty much anything well. He does a few things amazingly well, but
everything else he does at least very well. He has no weaknesses, no gaps in his ability. I don't think this was true of Nalbandian.
It is also interesting to compare Nalbandian and Nadal, because Nadal has been accused of being less skilled than his actual results would entail. But here's the thing: a large part of Rafa's greatness is his ability to come up with a two-handed backhand screamer across court, or one of those down the line forehands, or the right serve at the right time. Rafa is so money when it really matters - and that
is talent, that is ability, that is greatness. In fact, it may be that by comparing Nalbandian and Nadal, we have the perfect context to differentiate what makes a player great vs. what makes a player simply very good (and talented).