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Djokovic took almost three hours to beat the Italian 4-6 6-3 7-5, but will not be in Turin.

"I was really looking forward to competing in Turin and giving my best," Djokovic posted on social media.

"But after today's final in Athens, I'm sad to share that I need to withdraw due to an ongoing injury."

Djokovic also missed last year's ATP Finals through injury, having won his seventh title in 2023.

Musetti, who needed to beat Djokovic to overtake Canadian Felix Auger-Aliassime to qualify for the tournament, will replace Djokovic in the Jimmy Connors Group, with Alexander Bublik now the first alternate.

This marks the first time that two Italian singles players have qualified for the same edition of the ATP Finals, and is Musetti's first appearance.

Djokovic, at 38, became the oldest champion on the ATP Tour with his win in Athens, but he had to do it the hard way in a match that produced five breaks of serve in the deciding set alone.

The marathon encounter lasted one minute shy of three hours between the top two seeds, and Djokovic dropped a set against Musetti on a hard court for the first time as the Italian battled to win the match and seal his place at the ATP Finals in Turin on merit.

However, Djokovic recovered to win the second set, showing no signs of his advancing years when he stretched at the net to play a cross-court drop shot doing the splits at full stretch that got the crowd roaring its approval.

The drama reached fever pitch in the decider when an exhausted Djokovic was broken serving for the title at 5-4.

However, he rallied once again to break Musetti before finding his last reserves of energy to serve out the contest and eclipse Roger Federer by claiming a record 72nd hard court title.

Djokovic was initially too exhausted to celebrate but, after embracing the 23-year-old Musetti, the Serbian ripped his own t-shirt as he roared in delight.

“An incredible battle," he said after the win, without hinting that he would later quite the Finals in Turin.

"Three hours of a gruelling match, physically. It could have been anybody's match, so congrats to Lorenzo for an amazing performance. I’m just very proud of myself to get through this one.”

Djokovic is now two titles behind Federer in the ATP Tour's all-time list while Jimmy Connors leads the way with 109.

Musetti saved a match point in the semi-finals against Sebastian Korda, but couldn't capitalise on winning the first set against the former world No.1 Djokovic in the final.

“Novak, there are not many words to say about you and your career,” Musetti said after losing for the ninth time in 10 matches against the Serbian great.

“You are proving, still at your age, that you can kick us, like me today. Every time I share the court with you, I take it as a lesson, so thanks for that.”

Musetti, though, was gifted his spot in Turin later after Djokovic's withdrawal and will play in the same group as Carlos Alcaraz, Taylor Fritz and Australia's Alex de Minaur.