2008 - His first great season. Won the channel slam and the olympics. Reached #1 for the first time in his career.
2009 - Started the season really well, but could barely win sets against top 10 players after Rome. Failed to remain top of the rankings.
2010 - Sweeped the clay season and won 3/4 slams.
2011 - Consistently
by Djokovic, even on clay. Barely beat Federer
@ Roland Garros. Reached, but lost, some big non-clay finals because of favourable draws and weak competition.
2012 - Is playing much better and looks likely to win his 3rd channel-slam, defend his olympic gold medal and return to number 1.
This year-2-year inconsistency is unbecoming for a supposed all-time great.