With that logic we can say that for example Nadal peak was at 2008 and 2010 and Nole 2011 and rest of years when they won slams and were 'solid' and losing bit more was past peak?
Nadal suffered an injury in 2009 and had to take time out from the game. It took him a while to get back to his previous level of play. It's common for this to happen when you suffer an injury in sports. Forgive me if this sounds patronising but you did ask.
As for Djokovic, I think it's fairly clear to see he reached his peak in 2011. And what a peak it was. He's still pretty close to that level, no?