I just wanted to post in regards to Nadal's playstyle and the future health of his knees
A little background for this is that stressing the knees and working them out actually strengthens the cartilage, tendons, ligaments, and muscles in the knee, what causes injuries is overstressing the knee without leaving enough time for the knee to properly recover. If you look at other sports such as basketball and football which require sudden starts and stops and changes of direction as well, you'll see when the players get injured its from an acute injury such as tearing of a ligament or cartilage and that most players in these sports dont have the same tendonitis injuries a tennis player has.
A difference with Nadal seems to be his naturally strong physique, having strong muscles with high endurance reduces injuries as they act as stabilizers to control stress on the joint to something it can comfortably withstand. I think Nadal understands that he needs proper rest and along with federer plays a limited schedule now knowing hes going to be playing 4-5+ matches every tournament he joins, now with the way the ATP calendar works he cant really get enough rest he needs specially in certain parts of the season and there needs to be a change by the ATP in regards to this unless they want their players to have more injuries specially with the way the game is played today. But as to chronic knee injuries plaguing his career, I think the way Nadal balances his schedule and level of fitness, it he can keep his speed and endurance up he can continue to play the same high level for another 3-4 years which doesnt bode well for federer or sampras' records