Boris Becker says Novak Djokovic has two personalities: the Zen-like competitor, and the "sweetheart who would give you his last shirt if you needed one".

And Nikola Pilic, who coached the Serb in his teenage years, recalls a very high sport IQ and "incredible coachability".

As Djokovic prepares for next week's French Open, which could lead to a 19th Grand Slam title, BBC Radio 5 Live has spoken to three of the men who have helped him along the way.

The six-year-old boy, who decided he wanted to become a professional after watching Pete Sampras win Wimbledon from his parents' pizza parlour, has now been world number one for 322 weeks.

That is longer than any man in history, and just 55 weeks shy of Steffi Graf's all-time record.

'Incredible coachability' - Nikola Pilic

Jelena Gencic was running a tennis summer camp in the Serbian ski resort of Kopaonik, when she noticed a young boy watching intently through the fence.

She invited him to come and play the next day, and for the next half a dozen years would nurture Djokovic's career - as well as introduce him to classical music and literature.

It was just before his 13th birthday that Gencic, who died in 2013, decided Djokovic would benefit from a spell at Pilic's academy in Munich.

"He came with his uncle, and they came very early, and they jumped over the fence and they waited for me," recalls Pilic, a 1973 French Open finalist.

"He never forgot anything. Not towels, not a racquet, not balls - he was always there at least 20 minutes before training and he was doing his exercise and warming up before the start of play.

"The more I talked with him, I had a feeling that the guy is incredible at focusing on what he's doing. Even when we played soccer, and he was very good on that, too, he was everywhere where the ball was.

"I noticed in the early years that he had this sport IQ [that was] very, very high. The coachability of Novak was incredible."

He has 'two sides that fight with one another' - Boris Becker

About 13 years and six Grand Slam titles later, former world number one Becker received a call from Djokovic's agent.

It was late 2013, and Djokovic had just lost his number one ranking to Rafael Nadal and suffered four Grand Slam final defeats in 18 months.

Over the next three years, working with both Becker and Marian Vajda - a near-constant presence in Djokovic's life since 2006 - the Serb would win another six Grand Slam titles.

And during that time, Djokovic's German improved significantly as he took frequent opportunities to speak to Becker in his mother tongue.

"In a funny way, I would always call him 'schatzi', which means sweetheart, and so when people were in the elevator and he started picking up German, everybody was surprised," Becker recalls.

"He's got two personalities. There's the one on the court - the machine-like, Zen-like, businesslike competitor that wants to win no matter what. And then you've got the Novak off the court, who is a sweetheart - loves his family, loves his charity, loves his country - and he would give you his last shirt if you needed one.

"So you have these two sides that fight with one another sometimes on the court and I think that's why people sometimes don't understand or criticise him because they see this fierce competitor that can be ugly at times. But it all comes from a good place."

Djokovic, Nadal and Roger Federer could, in theory, leave the sport with 20 Grand Slam titles each. And Becker, who himself won six, is one of the few former champions prepared to say who he currently considers the greatest.

"Nadal is ahead, in my book, because of the way he plays the game," says the German.

"I would rate Federer as the most talented I have ever seen, I would rate Djokovic as the most fierce I have ever seen, but Nadal, boy, when he gets going and picks his shorts and the eyebrows, I wouldn't want to play him on any surface."

Coaching him 'can be 24-hour stress' - Goran Ivanisevic

A few months before winning Wimbledon 2001 as a wildcard, Ivanisevic was asked to practise with a 13-year-old Djokovic.

"We played a set, and then he gave me some chocolate because he thought I lost my energy," Ivanisevic says.

"He was special. He had something that you cannot learn, or you cannot buy - you are born with that.

"He was not even kind of impressed. He played like he was playing with his older brother. I'm always going to remember his eyes, and back then he believed that he can beat me."

Ivanisevic is now in Djokovic's coaching corner, trying to help him catch Federer and Nadal on 20 Grand Slam titles.

"To be part of the team, you have to be quick - to think quick," Ivanisevic continues.

"You have to always have prepared answers, and you need to be able to follow him. He likes to study, and sometimes you need to be in front of him, because if you are not ready and prepared, he just runs over you.

"Sometimes it's not easy, to be honest. It's a big challenge, and it's an honour to be there, but it can be 24-hour stress. It's not healthy, definitely, but when you see results, when you see that you are part of something which can make history, it moves you forward."

At last year's French Open, Ivanisevic was adamant Djokovic was going to beat Nadal in the final. This time round he sees it a little differently, but believes his man has been the best player in the world for the past decade.

"In my opinion, Nadal is number one favourite to win the French Open and there's only one person who can beat him to that number 14 title - and that's Novak," he says.

"But he has to be 100%. We saw last year he did not show up for the final. He showed up too late. You cannot give Nadal too much space, otherwise he eats you.

"If you take the results from the last 12 years, from 2009, he won by far the most Grand Slams, he broke all the records. I think he's the best.

"There's always going to be a debate, but if you really take the results in the last 12 years, by far - not little - by far Novak is the best tennis player in the world."

You can hear more from Becker, Ivanisevic and Pilic as BBC Radio 5 Live discusses what makes Djokovic the champion he is - and whether he gets the credit he deserves - from 21:00 BST on Thursday, 27 May, or later on the BBC Sounds app.