Reigning world number one Djokovic has already established himself as one of the best ever to play the game and the Serb's Grand Slam haul of 17 is just three behind the 20 that Roger Federer has amassed. The Swiss player will be 39 in August.
While Federer is nearing the end of a glittering career, Spanish left-hander Rafa Nadal is just one behind with 19 majors and will be 34 next month. Djokovic is the youngest of the so-called 'Big Three' and will be 33 later in May.
The triumvirate have shared the last 13 Grand Slam titles, re-energising the GOAT (greatest of all time) debate.
Kyrgios upsets Hurkacz by racing through service games in defeat at Montreal Open
- Djokovic: Positive emotions can purify polluted water
- Australian Open at risk due to COVID-19, says Tiley
- Pospisil hits out at Kyrgios for ATP-WTA merger opposition
"Roger is the most aesthetically pleasing I have ever seen," Martin, who was part of Djokovic's coaching team briefly in 2009 and 2010, told Tennis365.
"He is pleasing with the serve, he is pleasing with the return, offence, defence ... everything he does really looks beautiful.
"For me though, if Novak is right, if Novak is confident and comfortable with his physical status and is really focused, I've never seen anybody more unbeatable. This guy has played the game better than anyone else ever has, in my opinion."
Djokovic was in imperious form before the novel coronavirus pandemic brought the circuit to a halt in early March.
Image credit: Getty Images
He lifted the ATP Cup with Serbia, won an eighth Australian Open title and then completed a fifth triumph at the Dubai Tennis Championships, extending his unbeaten run to 21.
"His athleticism is from another world," said Martin, who reached two Grand Slam finals. "His return of serve is way better than any other returner of serve ever and I mean way better.
"Having worked with him, when he is focused he has that 'I'm going through a brick wall mentality'. Now he is not always focused, but when he is and we have seen this for long stretches, he doesn't have to play great. He is that much of a fighter and thrives on it."
Watch: Djokovic trains in violation of the rules of Spain's lockdown
The virus forced the cancellation of this year's Wimbledon for the first time since World War Two while the French Open was pushed back from its May start to September, shortly after the scheduled end of the U.S. Open.
Federer, who is currently ranked fourth, last played at the semi-finals of the Australian Open, where he lost to Djokovic, before undergoing keyhole surgery on his knee in February.
Martin said Federer was a "natural" and he still expected the Swiss to challenge for Grand Slam titles when the season is able to restart.
"With the Olympics being postponed, I would imagine it will compel him to play another year," Martin said. "Plus, this is no way to end anything. It is really important to Roger, I would imagine, to conclude his career on a bona fide high note.
"Nothing I've seen indicates that physically he is incapable of keeping going. Nothing I've seen has indicated his love for the sport has waned, so I wouldn't be surprised if we see several more years out of him."
Ruud fined for not going to the toilet during Montreal Masters bathroom break
Djokovic at US Open: America relaxes Covid rules - but star faces travel wait
Share this article