How, then, does Federer manage it? If tennis can be broken down into four elements - physical, technical, tactical and mental - then each piece of the puzzle plays a part. But the starting point must be his God-given gifts, which are hard to fully appreciate unless you are sitting beside the court.
"When I watch tennis, the sound tells me a lot," says Craig O'Shannessy, lead analyst for the ATP Tour and author of the Brain Game Tennis blog. "In Roger's case, the guy floats around so lightly, almost as if he's skipping rope, that you can hardly hear him. He has very elastic ankles and great strength in his calves so that the lower leg does most of the work. He's not carrying a huge amount of muscle - just the right amount to get the job done."
Tennis is an incremental game. Every movement is repeated ad infinitum, and Federer's elegant style - with its deft footwork and impeccable stroke mechanics - saves him a fraction of energy on every stroke. Those savings mount up over the years, which is why he is still going strong while contemporaries like Andy Roddick, Marat Safin, David Nalbandian and Lleyton Hewitt have either retired or are held together by steel rods.
"One of the keys to injury prevention is a smooth style," says Sergio Gomez-Cuesta, a biomechanical specialist at the Gosling International High Performance Centre in Hertfordshire. "Nadal is a magnificent athlete, but he wouldn't even be in the world's top 50 for smoothness. His backhand happens in a violent rush, with an incredibly fast swing. I don't know the details of his wrist injury but I do know that the body stiffens up as you get older and distal injuries [which occur at the outer end of bones] become more common in the wrists and elbows.
"If Nadal is all about force and power, then Federer has incredible finesse. He scores 10 out of 10 for smoothness, like Pete Sampras did, or John McEnroe - a guy who often looked as if he was hardly doing anything on the court, because he would feed off his opponent's pace. McEnroe is still playing at an amazingly high level today, aged 55, but you can't see Nadal having much of a senior career."
In tennis's greatest rivalry, opposite poles collide - which may help to explain its magnetism. Nadal uses his muscularity and endurance to wear down opponents from the baseline. The longer a rally lasts, the more likely he is to win it.
Federer's chief assets, by contrast, are his nimbleness and sudden speed. In fact, O'Shannessy believes he could have won even more titles if he had concentrated more ruthlessly on those key strengths.
"Over the past two or three years, everyone has been asking the same question: why doesn't he come forward more?" O'Shannessy says. "He was getting stuck at the back of the court and allowing opponents to find his backhand too often. It is only since he started working with Stefan Edberg that he has really made the most of his abilities by rushing the net more often.
"You go back through the history of the game and the best volleyers - whether you're talking about Edberg himself or Boris Becker or Sampras - have always had single-handed backhands. It's easier to manoeuvre the strings into position for the shot when you can use your non-dominant hand on the throat of the racquet. That's one advantage that Roger has over the crowd, so he might as well use it."
After a grim 2013, by Federer's own stratospheric standards, everything has come together this season. He is undoubtedly the form horse going into the US Open.
So, to return to the original debate, we can probably agree that Nadal at his best would beat a fit Federer on most surfaces. Even on grass, judging by the epoch-defining Wimbledon final of 2008.
But could Nadal ever be as great, for as long, as Federer has been? Not a chance.
And so to the follow-up question. Which is more important: the highest point of your most daring climb, or the length of time you spend in the mountains?
To that, there is no obvious answer.
