Novak Djokovic courted controversy Monday when he wrote "Kosovo is the heart of Serbia. Stop the violence" on a camera at the French Open after clashes flared in northern Kosovo amid ethnic tensions. The Belgrade-born tennis superstar wrote the message in Serbian after his opening round victory over Aleksandar Kovacevic on the showpiece Philippe Chatrier Court at Roland Garros. "Kosovo is our cradle, our stronghold, centre of the most important things for our country... There are many reasons why I wrote that on the camera," 36-year-old Djokovic told Serb media at the tournament.