Jimmy, this is an excellent post, and I agree with most of it. My quibbles are with what I bolded above. Nadal has lost something in speed, but he's still pretty fast. And strong, I think. But he's made a conscious decision to save his knees, for sure, and the rest of his body, by playing shorter matches when he can, via tactical changes, the endless work on the serve, and other ways you say that he's varied his game. It's a smart choice, it has helped him as the game evolves, and it has almost surely prolonged his career. As to the other bolded, and following on, I think he
can rely on his willpower (as
@Bonaca mentioned above,) which leads on to stamina, when required, such as in the recent Medvedev match, but he knows he doesn't want to/can't have to do that every match. I actually think his head, his determination, can take him farther than his body would like, but he has definitely realized where that leads. Starting even in '09, the body complains, and breaks down. I think he's been working on a better equilibrium ever since, but it seems that Moyà has been the kick in the pants that was really needed.