Nobody doubts Novak Djokovic more than himself.
Djokovic has won almost everything there is in tennis. No man has more Grand Slam singles titles or Masters 1,000 crowns than him. He will go down in history as one of the greatest players of all time, if not the greatest.
But even now, at the age of 39, that is not enough for the Serb.
"I cannot say I feel content with what I have and who I am," he says in a new Amazon documentary.
"I am always looking to be more - better, faster, stronger. Oftentimes I feel I am imprisoning myself."
He says later in the documentary: "I'm the number one doubter. I'm trying to prove myself wrong more than anybody else."
Djokovic is level with Margaret Court for the most Grand Slam singles titles in history, having won his 24th trophy at the 2023 US Open.
He has reached at least the semi-finals at six of the past seven Grand Slams and in January became the oldest man in history to reach the Australian Open final.
"He definitely has mixed feelings about his retirement," his wife, Jelena, says.
"'He carries that burden that he's never enough. No matter how much he does, it will never be enough.
"Tennis is a fulfilling place for him - a place where he feels worthy."
The documentary shows footage of a young Djokovic being interviewed on television. Asked how he feels about tennis, he calls it a "responsibility".
"My goal in tennis is to become a champion," he says.
Djokovic's career, from his style of play to his attitude on court, has been shaped by his childhood experiences in a way that few other players can possibly relate.
Born in Belgrade, Djokovic was a young child during the Nato bombings of the Serbian capital between March and June 1999.
He recalls running from his building - which did not have a bomb shelter - to another for safety. On the way, he tripped, and his unaware family continued running. Djokovic turned to look at his building and watched a plane drop two bombs on a nearby hospital.
"Pretty much every day, for years, we had to wait in line for bread and milk," Djokovic told BBC 5 Live in July.
"These are defining moments, something I want to portray as a building block for me as a player and a person.
"There is no regret or resentment. In contrary, it's appreciation and gratitude. Everything in life happens as a reason."
In an expensive and often elitist sport, the young Djokovic was an outsider. Djokovic speaks about his father borrowing money from "wrong people" to fund his early tennis career.
When he broke through, he disrupted the Roger Federer-Rafael Nadal rivalry that had come to engross fans. More emotive and more vocal than the pair on court, Djokovic was not universally beloved by fans.
"I had to suppress a lot of emotions which, later on, surfaced on the court," he told BBC 5 Live.
"On the court, that's where I felt safe and comfortable. It was a familiar ground.
"Tennis is much more than a game and competition. It is defining ground for who I am as a person."
Every Djokovic loss at a Grand Slam is followed by questions about his retirement. However his 2026 US Open campaign pans out, Djokovic will be ready for familiar talk about his future in tennis.
Long-time rivals Federer and Nadal ended their careers in 2022 and 2024 respectively, having struggled with injuries in their latter years.
Djokovic has limited his schedule in recent years, playing very few tournaments outside of the Grand Slams.
After losing to Sinner at Wimbledon last year, Djokovic said he was competing at the latter stages of Slams with the "tank half-empty".
However, Djokovic says he does not have "the expiry date of my career" in mind.
"Why would I talk about retirement when I still keep proving to myself that I can still win?" he says in the documentary.
"As long as I have the drive, the passion, but most of all, as long as I feel like I can be one of the best players in the world, I want to keep going."
Winning a long-awaited Olympic gold medal in Paris in 2024 was the pinnacle of Djokovic's career, and he is still aiming for the 2028 Games in Los Angeles.
"It seems a long way out. I feel like I can make it," he added.
"I would like to end with the Serbian flag on my shoulders.
"It would be a nice way to wrap up something I'm very proud of."
