Novak Djokovic, fresh from winning a record-equalling sixth ATP Finals title, said hunger for more success and love of tennis is what still inspires him to get out of bed.
The 35-year-old Serbian loves making history and his prime target is the overall record of Grand Slam titles, held by great rival Rafael Nadal with 22.
The pair should clash in the opening Grand Slam of the season the Australian Open in January. Nadal won this year's edition with Djokovic, who refuses to be vaccinated against Covid, being expelled from Australia.
Djokovic also missed the US Open because he is not vaccinated. He played Wimbledon and retained his title.
Nevertheless, as his body ages and the aches and pains become more a feature of daily life, Djokovic said he has to find a reason to rub the sleep from his eyes and jump out of bed.
"In those moments where you have pain, this or that, you don't feel maybe like standing up in the morning," said Djokovic after beating Casper Ruud to become the oldest player to win the season-ending tournament.
"There has to be something, whatever it is for all of us, that gets us up from the bed.
"It's the heart, it's the mind, whatever the life force or the fuel is for you."
For the intense Serbian the fuel is success.
"For me it's still the hunger that someone asked me about," he said.
"It's love for the game, no doubt.
"Passion. I love the game."
- 'Bad guy and the good guy' -
This love feeds into his desire to make history.
He has already made plenty, though he has said the record 373 weeks as world number one was the one closest to his heart and for which he "worked all his life".
"Of course, making history of the tennis sport, which of course is my favourite sport, the sport that has given me so much privileges in life and benefits," he said.
"Why not try it? Why not dream about it? I have no problem to verbalise that I have biggest goals, that I want to be the best, that I want to win every tournament.
"I don't think that's not humble. I just feel it's important to respect everyone in the game but still be confident with yourself. I don't see anything bad in that."
Djokovic said recovering after a tough match -- like his three-set battle which ended after just over three hours with Daniil Medvedev in the pool stage at the ATP Finals -- and preparing for another involves a lot of soul-searching.
"I think someone asked me after the Medvedev thriller, Where is the limit?" he said.
"It's an internal battle with myself because there's one voice that is always telling you: 'you can't do it, you're too tired', this and that, right?
"The bad guy and the good guy. You try to feed the good guy so he can become louder and stronger than the bad guy.
"It's as simple as that."