Nadal's defense is way overrated on hard courts, he is nowhere near guys like Hewitt, Djokovic, Murray, Simon on defense on hard courts. Nadal is not a counterpuncher, he cannot counterpunch well. The formula Djokovic uses is quite simple - exploit Nadal's weak defense on fh side and play inside the court, taking ball early. Djokovic also cover the court very well, using defense to take winners away from Nadal. It's a simple formula but not many can use it as almost no-one has the set of skills Djokovic has.
Notice how weak Nadal is as a defensive player on hards, when people attack his fh with flat shots, Nadal cannot counterpunch, he is late hitting the ball and hits ball back shorter and shorter. Djokovic exploits this better than anyone by using his bh cross court, attacking Nadal's fh consistently. Djokovic is very smart and can see how weak Nadal is on the defense off his fh side. Contrast this to a guy like Hewitt, who was a true counterpuncher and was able to use short backings to re-direct opponents hard shots for even winners. I remember i saw Hewitt dismantle Feliciano Lopez at US Open, i was at that match. It was remarkable how Lopez was going for big shots and Hewitt was just a wall, putting his racket on the ball and using Lopez' pace to hit winners! Nadal lacks this ability, he is truly a horrible counterpuncher. He can run well but he is horrible counterpunching.
Djoker also runs down all of Nadal's shots and take winners away but Djoker does this to everyone, expect one man - Wawrinka.
When a player gets Nadal on defense on a hardcourt, Nadal loses, as he sucks at counterpunching. It is when Nadal is on the offense that he wins on hards... whoever can take his offense away and put him on defense, beats him. Not many have the movement and ball string skills to do this..