Nadal's lefty massive, high-bouncing, topspin crosscourt forehand to Federer's one-handed backhand has always been a major headache for Federer. However, that same shot to Djokovic is ineffective because Djokovic effectively nullifies Nadal's lefty advantage with his great two-handed backhand, which he can crush either down-the-line or with sharp crosscourt angles. Thus, without his lefty advantage, Nadal is basically left without any weapons. He might as well play right-handed against Djokovic because the results would be the same.
And anyone who claims that if Nadal played right -handed he could just hit inside-out forehands to Federer's backhand and have the same results is wrong. That's because Nadal's inside-out forehand is much flatter than his cross-court forehand so that ball wouldn't bounce nearly as high to Federer's one-handed backhand. Federer would also have far fewer problems with Nadal's serves in the ad court.