The Serbian became the first man in 47 years to hold all four Grand Slams at once with his triumph at Roland Garros last year, but his title defence ended in scarcely-believable fashion as he was beaten in straight sets by the unseeded Austrian.
Djokovic was particularly poor in the final set as he won just eight points on the way to his first 6-0 loss in a Grand Slam set since 2005.
"I don't expect myself to play as bad as I played in the third. I don't visualise that ever. It was decided I think in the first set. I tried. I lost that crucial break in the beginning of the second, and he started serving better, backing it up with the first shot. He was definitely the better player on the court," he told reporters.
"It was not there for me today, especially in the second part of the match. I was just unable to hit the ball well and many unforced errors. The last couple of tournaments, I have had some great matches. It's unfortunate to finish Roland Garros the way I have done. I obviously always expect a lot from myself, but it's a fact that I'm not playing close to my best, and I know that.
"I love this sport. I'm motivated as any other player on the Tour. Even though I have played for many years, I still want to do well. Trust me, I'm thinking about many things, especially in the last couple of months. I'm just trying to sense what's the best thing for me now.
"Obviously there has been a lot of changes with the team and so forth. I'm excited to work with Andre (Agassi) and the new team. At the same time, I have a responsibility to the game itself."
The defeat also means that Djokovic will drop out of the world's top two for the first time in six years.