Former German tennis great Boris Becker thanked world number one Novak Djokovic Sunday for his backing while he was in a British prison and called the Serb "probably the greatest player of all time".

Presenting a new documentary about his on-court triumphs and off-court travails at the Berlin film festival, the 56-year-old said his former pupil had been a steadfast ally.

"Novak Djokovic became a family member. You know we had a professional relationship for many years and we parted ways in 2016," Becker told a news conference.

"But we always stayed close and especially my time inside (prison) -- he supported me, he supported my family."

From 2014 to 2016, Becker coached Djokovic to six of his 22 Grand Slams -- the Serbian said he was "heartbroken" when his former coach was sentenced to prison.

 

- 'Tennis winning machine' -
 

Djokovic talks in the documentary "Boom! Boom! The World vs. Boris Becker" by Oscar-winning director Alex Gibney about learning about "mental strength" under pressure from Becker while he coached him.

Becker, a six-time Grand Slam champion and the youngest Wimbledon winner at 17, thanked tennis legends John McEnroe and Bjorn Borg for participating in the film along with Djokovic.

"I'm very proud that probably the greatest player of all time is part of a story of Boris Becker," he said of Djokovic.

Becker credited Djokovic's rivalry with Rafael Nadal with helping to revive global tennis -- the Serbian's victory at the Australian Open last month drew him level with the Spaniard on a record 22 singles Grand Slam titles.

Roger Federer, the third of the "Big Three" who have dominated men's tennis for the past 15 years, retired last year with 20 Grand Slam titles.

"Tennis has two kings at the moment which is great," he said. "This is why tennis at the moment is again very exciting."

Becker was deported to Germany from the UK in December after serving eight months of a two-and-a-half-year jail sentence for flouting insolvency rules by hiding £2.5 million ($3.1 million) of assets and loans to avoid paying debts.

Since regaining his freedom, Becker has said he relied on "blood brothers" to protect him in prison and revealed his life was threatened twice behind bars.

The first of two parts of the documentary for Apple TV+ premiered at the Berlin festival, Europe's first major cinema showcase of the year.

In the film Becker is seen tearfully accepting his conviction and wrestling with his life's missteps.

Becker said his tender age when he became a global superstar and his reliance on reckless advisors had set him up for failure down the road.

"Life as a tennis winning machine, it's a lot harder than it looks," he said.

"We travel every week to another city, another country, another continent and then we have to function. Every player has a different way of dealing with these expectations, these pressures and always trying to win."

Becker admitted he had "weaknesses and some dark moments" but said he thought his tumultuous life lent itself to cinema.

"My life seems like a movie," he told reporters. "It just happened to be real."