Novak Djokovic failed to train hard enough and will not regain the No 1 world ranking "unless he puts in the hours", former coach Boris Becker said on Wednesday, a day after the pair ended their working relationship.

Djokovic has won 12 Grand Slam titles - six under Becker - but on Tuesday the Serb confirmed rumors that he and the German legend had parted ways after three years together.

Becker said Djokovic's need to spend more time with his family had derailed him on the court.

"He has not spent as much time on the practice court as he should have in the last six months and he knows that," Becker, 49, told Sky Sports News.

"Success like this doesn't happen by pushing a button. Success like this doesn't just happen by showing up at a tournament.

"You have to work your bottom off, because the opposition does the same," he told Sky News.

"The profession of a tennis player is probably the most selfish one in sports because it has to be all about you.

"Novak is the first to say he is a family man, so of course his wife and the rest of his family had to take back seats.

"They don't spend enough time together. I had it too, 20 years ago. It is just the nature of the beast, being a tennis player."

After winning the French Open in June, Djokovic had a poor second half of the season, losing the No 1 ranking he had held for 122 weeks to Britain's Andy Murray in November.

Djokovic also lost his Wimbledon and US Open titles and was eliminated in the first round of the Rio Olympics.

"I know the US Open loss in the final against Stan (Wawrinka) hurt," Becker said.

"I think what he needed, maybe in a funny way, was to lose a little bit, to realize what it is like to lose, because he hadn't been losing for more than two years.

"I am sure the fact that he lost the No 1 ranking to Andy is going to hurt, but he has got to go back to work. He has to go back to the office and practice for hours and refocus on what made him strong in the first place."

Djokovic made a change to his coaching team toward the end of the season when Spaniard Pepe Imaz joined him for the Paris Masters, where the four-time champion fell in the quarterfinals, and the World Tour Finals in London - where he lost to Murray.

"Such a decision does not happen overnight, it was a process," said Becker, calling their separation "consensual".

Becker, a three-time Wimbledon winner, said he will now be Djokovic's "greatest fan" and believes the 29-year-old will regain his No 1 status.

"I am also convinced that he will become the most dominant player again, but he has to get back on the practice court and put in the necessary hours," said Becker.