Actually it's easy when your serve doesn't let you down. The serve is the most important shot in the game, and Fed's serve was brilliant in that match. He kept Nadal guessing, and Nadal's return game really suffered from it. Fed also went for his shots without fear. Sure, some of them were unforced errors, but some were winners, and some forced Nadal to errors. Finally, Federer also excuted the dropp shot to perfection. The best drop shot in the sport by far, as proven both aginst Del Potro and against Nadal.