Jannik Sinner and Carlos Alcaraz are poised to add another entry into their rapidly developing rivalry when the ATP Finals open Sunday without any members of the Big Three for the first time in nearly a quarter century.

Defending champion Novak Djokovic pulls out of the ATP Finals because of injury, a move which sets the eight-man field for the event.

Novak Djokovic will not defend his title at this year's ATP Finals as he's withdrawn from the tournament due to an unknown injury.The 24-time major champion posted the news to his Instagram story on Tuesday.

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.”

Citing an unspecified, "ongoing injury," Novak Djokovic announced his withdrawal from the season-ending ATP Finals on Tuesday.

Novak Djokovic has pulled out of the season-ending ATP Finals due to an injury, the record 24-times Grand Slam champion said on Tuesday. The 37-year-old Serbian also withdrew from the Paris Masters, which was won by world number two Alexander Zverev on Sunday.