im still believing this is fake
A tennis pro, let alone one of the best tennis stars to ever play the game, can certainly do that. I practiced a lot back in the day, and during practice, coaches would set up a pyramid of tennis balls in different positions in or around the respective service box. The objective was to hit the targets with 1st service-quality serves. If you hit the targets, you get to go on water break, if not, you'd have to run a set of lines. Again, someone of Federer's level of skill, would definitely be able to something like this repeatedly. I'm sure he'd miss, but wouldn't be too far off.