Rafael Nadal has been fortunate to win 2 majors on hard courts in his career. At the AO in 2009, he somehow amazingly recovered from an exhausting 5+ hour 5 set match with Verdasco where he won when Verdasco basically broke himself by double faulting twice, including match point, at 4-5, and then went on to beat Federer a day and a half later also in 5 sets. Then he just had to jump on the hard courts at Rotterdam, didn't he? The damage was done, and though he managed to win 3 of the lesser clay tourneys, look what happened at Roland Garros and Wimbledon that year.
A year and a half later, he completed his best slam year by winning the US Open over Djokovic. During this tournament, he came up with an amazing serve and one might say his draw was favorable. In 2012, he played the longest AO match in history with Djokovic where they almost both collapsed. didn't play again till Indian Wells, where he lost to Federer in the SF, but won the doubles title, then at Miami he had to withdraw due to knee problems to Murray in the SF. Again, the damage had been done, though he was able to last through Roland Garros until Wimbledon.
He has won 8 majors on clay and 2 on grass. So 83% of majors were won on natural surfaces. A high percentage of his 58 titles have come on clay - 42, 3 on grass, so 45 of 58, close to 78% titles won overall on natural surfaces.
Now ask yourself, as he approaches his 10th year playing at slam winning level, and considering the competition, on which surface(s) do you think he has his best chances to win a major, or any other title? Even though he has won 13 titles on hard in a nine year span including 2 this year, showing he is not an incompetent player on the surface, it is still obvious where his best results and talents lie.
Again, though his play has ended up being disrupted due to his knee problems after Roland Garros/Wimbledon, the big problems with the knees always began with the hard courts in the early part of the year. He was able to manage the problems with his knees via shots and other therapy and keep himself going until after Roland Garros. Of course, if he started the clay season with problems, going deep in almost every clay tournament with the problems, also took its toll.
This makes sense when you consider that the Australian Open, Indian Wells, and Miami are probably the slowest, high bouncing surfaces, which equals longer rallies, longer games, and longer sets on the joint jarring surface. Even though these conditions favor Nadal's game style, it is murder on his knees. Resting for 7 months, and then in 2013 only playing only 1 hard court tournament - Indian Wells, prior to Montreal, is an investment that has probably helped him be healthier now than he has been in quite a while. Notice he wore no tape on the knee in Montreal.
In order to extend his career as much as possible and be successful, I think he must have been advised to avoid the hard courts as much as possible. Play only the required number of masters, majors, YEC, and possibly a tune up. Refrain from playing two hard court masters two weeks in a row in slow conditions, that is, visit his friend Larry Ellison and play Indian Wells and skip Miami. And only do this amount of hard court play if close to 100% healthy. Using this strategy might get him to 2016 in reasonable shape. Playing 5-6 hour best of 5 matches at the AO and 3-4 hour best of 3 matches at Indian Wells and Miami and other insignificant hard court tournaments is a recipe for problems.
Respectfully,
masterclass