Its really hard to say because both are struggling. I think Nadal will make to final whereas Federer will be in the same half of Djokovic draw as-usual (unfortunate because their 2 semi's last year was better than final). So Federer will lose to Novak again.
i think murray will have great tournament.
i will say Del Potro will be a threat since its hard court where he is at his best I don't think he will be threat to Novak but Federer and Nadal sure don't want to see his name in their quarters of the draw.