Boris Becker says constructive criticism helped Novak Djokovic's resurgence.
The truth hurts—and helped Novak Djokovic regain his edge.

Former world No. 1 Boris Becker, Djokovic's former coach, says blunt constructive criticism sparked the Serbian during their three-year partnership.

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In an interview with Marca, Becker said he sat down with Djokovic for a 48-hour deep discussion on tennis before signing on as coach.

The Hall of Famer revealed he delivered two criticisms directly to Djokovic: complacency and a unimposing serve were preventing him from fulfilling his potential.

"I went to see him in Monte Carlo and for 48 hours we only talked about tennis," Becker said. "I told him that I was always going to be very honest with him if things were not going to work: The reason why you can't be better is because you got comfortable being number one and you didn't improve. Both Nadal and Federer have. I don't like your position on the track and I don't like your service either."

Djokovic began working with Becker in December, 2013. The Belgrade-born baseliner won six Grand Slam titles during the pair's three-year partnership.

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These days Becker spends time overseeing the German Davis Cup squad and providing commentary for Eurosport and says he'd be open to returning to coach under the right circumstances.

"I do not rule out that option, but right now I could not because I am responsible for the German Davis Cup team," Becker told Marca. "But nothing is eternal. If there are players that interest me, that allow me to train and want to be open with me, then that is something I could definitely consider.

"The problem with training is that it takes up a lot of time. When I collaborated with Novak Djokovic, I spent 25 to 30 weeks with him. That includes the preseason and smaller tournaments. I have a family and I have other professional projects to worry about right now. But I don't want to rule out the option to train because tennis is my great love."