BELGRADE, Serbia — Novak Djokovic won't defend his ATP Finals title after ruling himself ... Djokovic finishes the year with a 37-9 record but only one tournament championship: his first Olympic gold medal, which came at the Paris Games in August.

Djokovic has played in only two events since his disappointing third-round exit from the US Open. He reached the final of the Shanghai Masters but was defeated by Jannik Sinner, before losing against the Italian again in the Six Kings Slam.

Novak Djokovic won't defend his ATP Finals title after ruling himself out Tuesday due to an unspecified injury. Djokovic wrote in an Instagram story, "I was really looking forward to being there, but due to ongoing injury I won't be playing next week.