Novak Djokovic was absent from the ATP Finals and the Serbian's return to action at the Australian Open could be tougher than first thought.

BELGRADE, Serbia (AP) — Novak Djokovic won’t defend his ATP Finals title after ruling himself out on 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. Apologies to those who were planning to see me.”