Despite a combination of age, injury and long-held prediction of a new generation of stars about to burst through, Djokovic insisted the four veterans remained the players to beat.

"I think if we are healthy and playing well, the four guys still have probably the best chance to always win Slams," said Djokovic in Doha, ahead of playing in his season-opener, the Qatar ExxonMobil Open 2019.

Djokovic is the favourite to dominate in 2019, having finished last season so strongly, overcoming an injury-hit start to the year to claim the Wimbledon and US Open titles.

The 31-year-old also rose from world number 22 to number one, becoming the oldest ever player to finish the year as the highest ranked player in the world.

His rivals, especially Nadal and Murray, are still battling back to full fitness, increasing the chances that the Serbian could add to his 14 Grand Slam titles.

One more major win and he will overtake Pete Sampras to become third on the all-time list of Grand Slam winners. Only Federer (20) and Nadal (17) have more.

Djokovic though insisted that "the next generation is already there" and singled out Germany's world number four Alexander Zverev, Croatia's Borna Coric, number 12 and Greece's number 15 Stefano Tsitsipas as the main threats.

"It's a matter of time when we will see some of them competing in the last stages of Grand Slams," he added.

Djokovic won a Qatar Open doubles match on Monday, partnering his brother Marko.

In Doha, his main challenge is likely to come from Karen Khachanov who ended Djokovic's 22-match winning streak in November and number two seed Dominic Thiem.