IMO, the only player that can beat Novak right now is himself. He sometimes goes thru this phase where nerves get the better of him - like leading 4-0 then lose the next 4 games to Rafa.
Rafa's game has not really decline that much, it is those other players who have stepped up theirs. Roger had reinvented himself to improve his serve and volley. Andy is starting to trim his temper, improve his second serve and endurance to withstand long rallies and multiple sets. Stan now has the most devastating single-handed backhand in the the tour. Rafa has to bring in something new, maybe a stronger first serve or a new coach.