Novak Djokovic's former coach Boris Becker says the Serbian star lost his momentum in the second half of last season and was toppled as the tennis world number one by Andy Murray because his work-rate dropped.
Speaking a day after the two terminated their cooperation by mutual consent, Becker said Djokovic's need to spend more time with his family had derailed him on the court.
"He didn't spend as much time on the practice court in the last six months as he should have and he knows that," said Becker, who helped Djokovic to six grand slam titles after they teamed up in December 2013.
"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 said.
"The profession of a tennis player is probably the most selfish one in sports because it has to be about you and he 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."
Having completed a career slam when he won his maiden French Open title in June, Djokovic suffered an astonishing loss of form.
He crashed out of Wimbledon in the third round to American Sam Querrey and made a first-round exit at the Rio Olympics at the hands of Argentine Juan Martin Del Potro.
Djokovic then lost the US Open final to Swiss Stan Wawrinka, a defeat six-times major champion Becker said was a massive psychological blow to the Serb.
"I know the US Open loss in the final against Stan hurt," he said.
"I think that is what he needed maybe in a funny way was to lose a little bit, to realise what it is like to lose, because he hasn't been losing for two and a half years.
"I am sure the fact that he lost the number one ranking to Andy Murray is going to hurt.
"But he has got to go back to work. He has to go back to the office and practice these hours and refocus on what made him strong in the first place."