The only player who plays against Novak that you feel has a good chance of beating him is Stan at this point. He has the weapons to hit through his brilliant defence and he always brings his best against Novak. Federer doesn't have the capability to beat him over 5 sets anymore, Rafa is not the same anymore and Murray seems to shrink mentally when they play. Nishikori is somebody who could threaten him, but he's injured too often