Ex-coach of Steffi Graf believes Novak Djokovic can win 'three or four more Grand Slams' before ending astonishing career

Novak Djokovic could end his career on 28 Grand Slam titles, that is the view of former coach of Steffi Graf, Heinz Gunthardt.

Although Djokovic is in his mid-thirties, he is still producing on a yearly basis, this is helped in part too by his scheduling which often sees Djokovic only play what he needs to and keep himself refreshed for Grand Slam title tilts.

But Gunthardt has issued a warning that Djokovic will end up getting to a point when experience is good, but age can catch up with him and he won't be able to produce quite like he used to.

But when asked if he could dominate for another three to four years, he believes that he can of sorts but that a decline also can occur quickly.

"Three or four years is still a very long time. I still think he can win three or four Grand Slams. In the end, it often happens very quickly," said Gunthardt to Tennis Magazin.

"If you look at Nadal winning the Australian Open last year, it was one of the greatest achievements in sports history. But achieving something like that requires additional energy, which as an older player you can no longer replace after a certain day.

"Experience shows that it happens really quickly. You maintain and maintain the level - and suddenly it doesn't work anymore. You slow down, there are always twenty or thirty centimeters missing. That doesn't sound like much, but it's worlds apart.

It makes a difference whether I can accelerate the ball myself or just hobble around on the defensive end,"