Quite simple. Talent. He has the talent to make small changes to his game and suddenly he becomes good enough to win. How many players, when they get to 35/36, can make adjustments to their games and make up for the inevitable degeneration in reflex, agility, strength, and athleticism in general to compensate how much of losses they experience? That's exactly what Federer has done this year. He is not as fast or strong as he was 10 years ago, but he is taking the ball even earlier and has become even more aggressive with his approach. He hits his BH with more brief but compact swing with bigger frame now. He hits returns with even earlier contact and gets into attacking position straight away. And all these changes are possible because he has the talent to do it. Let's admit this - Federer's success at this advanced tennis age is only possible because of his talent. You can work as hard as you want to, but without the talent, it's just not possible to stay this good at his age and that's simply why not many players can play as well at 36.