How does Novak's 2015 season compare to his 2011 season?

Novak Djokovic had a fantastic 2015 going 83-6 on the year including 27-1 in the Grand Slams.  But he had a fantastic 2011 as well, so since I'm now calculating Estimated Dynamic NTRP ratings for ATP matches, I thought it would be interesting to compare the charts for each year.

Here is 2011.

Clearly a strong year with a rating over 7.0 much of the year, but he didn't finish strong with a couple losses to Ferrer and Tipsarevic at the Tour Finals and some other close wins against weaker opponents late in the year and a loss to Nishikori.

Here is 2015.

While he finished the year strong, and was over 7.0 several times during the year, he was not as far over 7.0 in 2015 as he was in 2011, nor was he as consistently over 7.0.

Since the data is connected, the relative ratings between years should have some meaning, and this would seem to indicate that perhaps his 2011 was stronger than 2015 through the majors, but just slipped a bit at the end.

Since Novak was generating higher ratings in 2011 with a similar record, one could perhaps come to the conclusion that the opponents were not as consistently strong.  There are many variables with draws and who he plays in early rounds, or if seeds are upset early and Novak doesn't have to play them late, that can influence this too, but with this many matches played, this data at least would seem to support the notion that 2015 had a slightly weaker top-50 to top-100 than 2011 did.

Attribution: Match data is courtesy Jeff Sackmann / Tennis Abstract.