Novak Djokovic has pulled out from the Monte Carlo Masters because of a knee injury.
The second-ranked Djokovic, a finalist in 2009, said in a statement that his withdrawal is a ''tough blow'' because the Monte Carlo Masters is ''his favorite tournament.''
Djokovic was set to start his clay-court season at Monaco next week. He didn't say how long he will be sidelined.
The Australian Open champion has had a stunning start to the season and is 24-0 in 2011.
Djokovic has won 59 of 65 sets and four tournament titles on three continents.