World No. 2 Novak Djokovic announced Friday to pull out of the Monte Carlo Masters tennis tournament, citing a knee injury.

"It's a tough blow and sad news not only for this tournament and my fans, but also for myself," said the Serb.

"I practice all year long on the courts at the Monte Carlo Country Club and it's unfortunately with great regret that I've taken this decision to pull out," added Djokovic.

Rafael Nadal, beaten by Djokovic in Indian Wells and Miami recently, is gunning for a seventh title on the clay of Monaco.