After splitting with Novak Djokovic, Boris Becker said the former world number one was not practising as hard over the past six months.
Serbian star Djokovic and coach Becker parted ways on Tuesday after three years and six grand-slam titles.
With Becker in his coaching box, Djokovic doubled his tally of majors to 12, while victory in this year's French Open saw the 29-year-old complete a career Grand Slam and hold all four of his sport's premier singles titles at once.
But Djokovic suffered a dip in form as he surrendered the ATP singles top ranking to Andy Murray.
And while he enjoyed three successful years in his corner, Becker - himself a six-time grand-slam champion - admitted coaching Djokovic had become more challenging.
"It was mutual," Becker told Sky Sports. "A decision like this does not happen overnight. It is a progress.
"I think the last six months have been challenging on many levels. Our hands were tied a little bit because we couldn't do the work we wanted to do.
"He didn't spend as much time on the practice court in the last six months as he should have and he knows that.
"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."
"I don't know if he had any personal problems based on what I know," Becker continued. "He is happily married. He has got a beautiful son.
"But 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.
"That can't be forever and I think that is what he meant. I don't think there were problems. I have met his wife - she is lovely and very, very supportive of her husband.
"But 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."
Despite being overhauled by Murray, Becker is confident Djokovic will return to the summit of men's tennis in 2017.
"I am sure the fact that he lost the No 1 ranking to Andy Murray is going to hurt," he said. "I know the US Open loss in the final against Stan [Wawrinka] hurt. 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'm convinced - and I am his number one fan for next year - that he will come back and regain that No 1 position and regain being the most dominant player in his sport.
"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."