Federer in 9.
After Novak gets less able to run down every single shot, it will happen eventually, it will become about skill. After set 6-7 they'll both be playing like elderly versions of themselves.
Novak has little vs. Federer if you take away his ability to run down shots and his athleticism advantage. Fewer weapons at that stage and less able to play with touch.
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM