"I don't like your serve at all" - Boris Becker to Novak Djokovic when they first teamed up

Honesty and trust are the building blocks for any successful player-coach relationship. And that is what six-time Grand Slam champion Boris Becker tried to establish before taking on the job of Novak Djokovic's coach, as he bluntly criticized the Serb's serve and overall game.

Becker and Djokovic joined forces at the end of the 2013 season, when the Serb was trailing Rafael Nadal in the race to be the world's top-ranked player. The three-year association would blossom into one of the biggest success stories in tennis, yielding six Grand Slam titles - one of which was the long-coveted French Open in 2016.

In a chat on Eurosport Germany’s Vocal Athletes podcast, Becker talked about how the two put together such a glorious journey. It all started with the German being brutally honest, even if that meant upsetting Djokovic.

"My promise to him was that I had to tell him open and honestly the truth, because everything else doesn't work and I can't embellish anything," Becker said.

Stuck at six Slams back then, Novak Djokovic was desperate for a new voice to inject a fresh approach into his game. The now eight-time Australian Open winner visibly struggled throughout the 2013 season, and kept faltering at the last hurdle of Slams.

He also had to relinquish his World No. 1 ranking to Rafael Nadal, who was having one of his best years on tour. In that context, getting Becker on-board was a game-changer for Novak Djokovic.

You didn't improve after becoming No. 1: Boris Becker to Novak Djokovic

Novak Djokovic practising under the watchful eyes of Boris Becker

When Becker joined Djokovic's team, he immediately pointed out that his serve is his biggest liability. The three-time Wimbledon champion also did not shy away from telling him that he had become complacent after reaching the pinnacle, which had stopped his growth.

"The reason why you might not be so good anymore is because you felt too good as number one, you didn't improve. The other players like Nadal and Federer have adjusted their game. I don't like your positioning on the court, I don't like your serve at all," Becker told Djokovic back then.

Novak Djokovic asked Boris Becker for help at the Slams even after split

Novak Djokovic and Boris Becker celebrating after the former won the 2016 French Open

The current World No. 1 did listen to Becker, and that resulted in him fortifying his serve and sprucing up his volleys. Their combined efforts culminated in Novak Djokovic winning four Majors in a row - from 2015 Wimbledon to 2016 French Open.

However, the Serb's elbow injury flared up after that winning run in Paris, which signaled that the partnership had run its course. The pair parted ways at the end of the 2016 season, after what Becker claimed was "the time of his life".

But the relationship remains solid as ever even today. Becker confirmed that he has always been there for the 17-time Grand Slam champion whenever he has come to him with a query.

"At the subsequent Grand Slams we always met whenever he had a question about a player," Becker said. "My door was always open. A player-coach partnership like that is above all characterised by trust. I can only help a player if he openly addresses his worries and fears. He has to be totally honest and I really appreciate that trust."