World number two Novak Djokovic, unbeaten this year, said on Friday he was pulling out of next week's Monte Carlo Masters due to a knee injury.
"It's a tough blow and sad news not only for the tournament and my fans but also for myself," the 23-year-old Serb said in a statement.
There were no further details about the extent of the injury.
Djokovic, who lives in the principality and trains for large parts of the year on the Monte Carlo courts, was hoping to launch a successful clay court season and extend his unbeaten streak of 26 matches dating back to December.
Since winning his second grand slam at the Australian Open in January, Djokovic has triumphed in Indian Wells and Miami and is putting pressure on top ranked Rafa Nadal to remain at number one.