Berlin - Novak Djokovic failed to train hard enough and will not regain the top world ranking unless he "puts in the hours", former coach Boris Becker says, after the pair split.
Djokovic has won 12 Grand Slam titles - six under Becker - but on Tuesday the Serb confirmed rumours that he and the German legend had parted ways after three years together.
"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 doesn't come by pushing a button. You have to work your butt off because that is what your opponents are doing."
After winning the French Open in June, Djokovic had a poor second half of the season, losing the world number one 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.
He made a change to his coaching team towards the end of the season and Spaniard Pepe Imaz joined him for the Paris Masters, where the four-time champion fell in the quarter-finals, 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, says he will now be Djokovic's "greatest fan" and believes the 29-year-old will regain his number one 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.
"Novak must concentrate on what has made him strong."