Novak Djokovic writes message about Kosovo after French Open victory
Djokovic's message reads 'Kosovo is the heart of Serbia. Stop the violence'
Clashes between NATO peacekeeping soldiers and Serb protesters result in injuries
NATO chief Jens Stoltenberg calls on Kosovo to tone down tensions with Serbia
Serbia and Russia do not recognize Kosovo's independence, impede UN membership
French tennis federation will not make a statement on Djokovic's message