He's entered the decline phase of his career seeing as he could only defend one of three slams, the opposite of Federer during his peak years circa 2004-2007.
More like Federer in 2008 and Nadal in 2011. Slams won't come as easily in the future as they did for Djokovic in 2011, his peak year. He will soon hit 600+ matches which is when players begin declining as seen in the past 20 years.
Interesting theory, this. In 2008 Federer was knocked off his perch by Nadal, and then Nadal by Djokovic in 2011. So who will knock Djokovic off his perch in 2013, as it were (assuming he ends this season as n.1)? Murray?? Is Murray not also near to reaching 600+ matches and therefore in decline