Why wouldn't Federer feed Nadal exclusively with slow-paced deep balls, sharply angled from time to time, but never hitting the ball hardly? Then Nadal would have to make all of the game and he has much problems to 'kill the ball', and he would probably start to make more mistakes/tire down. Federer should also make Nadal to run a lot and expecialy make sure Nadal hits a lot of jumping two-handed backhands - Nadal would become more and more tired it would be more and more easy for Fed to close out the match. But he should continue playing patiently this same way every game of the match, however long the match would be, because Nadal feeds on tempo and fired-up opponent and instantly comes to life when the other guy livens up! In Federer's only win against Nadal on clay, Hamburg 2007, Federer aplied similar strategy!
Of course this has little to do with other surfaces? but who knows, maybe it would work on them to
Are you joking? In Federers win over Nadal at Hamburg the match only turned around early in the second set when Federer looked like thinking "%¤#% it, if nothing else works, at least I can hit a few winners", and basically started hitting winners from everywhere on the court. In fact, a common trend in their matches is that Federer only has the upper hand when he plays very aggressively and does it well. As soon as he becomes hesitant he loses - quickly!