Novak Djokovic has been burned by high expectations at the Olympics before, so it was little surprise that he should express only guarded optimism on the eve of the Tokyo Games.

The Serbian, who has won the Australian Open, French Open and Wimbledon already in 2021, is chasing the gold medal that would leave him one step away from the first tennis calendar 'Golden Slam' by a man.

Should he triumph in Japan over the next fortnight, Djokovic will head to the US Open in late August needing to win in New York to complete the full set.

Steffi Graf, who in 1988 won all the slams and gold at the Seoul Olympics, is the only player in tennis history to have completed such a sensational season.

Djokovic said such a feat was becoming a "more and more realistic" target, as he spoke in a Serbia team news conference, but the last time he stepped off the singles court at an Olympic Games, he was in tears, having lost in the first round at Rio to Juan Martin del Potro.

There is no danger of Djokovic running into his Olympic nemesis in Tokyo, with Del Potro, who also beat Djokovic in a bronze-medal match at the London 2012 Olympics, not a part of Argentina's squad as he battles back from four knee operations.

Looking at his own prospects for Tokyo and the rest of the year, Djokovic said: "It is still a long way to go ... I put myself in a very good position. But I will take things very slowly and cautiously.

"It's an approach I need to have because in the past I wasn't probably fully experienced in that approach. And I started to feel there were a lot of distractions around that influenced my performance.

"I know there are a lot of things on the line, a lot of history on the line. I'm privileged to be in this position. I worked very hard to be here, with my team, of course. But let's talk about history if everything goes great."

After his singles loss in Rio, Djokovic and Nenad Zimonjic lost in the second round of doubles to Brazilians Bruno Soares and Marcelo Melo.

A bronze from Beijing 2008 is all Djokovic has to show for his Olympics career to date, but he is a hot favourite for the title in Tokyo, where he will start against Bolivia's world number 139 Hugo Dellien.

The 34-year-old Djokovic said he "would be delighted to ask" Graf how she achieved her 1988 clean sweep.

"When I was thinking about her ultimate achievement ... I did not think it... I don't want to say 'achievable', but [I thought] there was a slim chance that someone could make it again, both male and female," Djokovic said.

"But right now it seems more and more realistic. Of course, it is one of my goals and dreams."

Djokovic has neither Roger Federer nor Rafael Nadal as rivals in Tokyo, with Federer missing after reporting a knee injury setback and Nadal electing to give the event a miss.

The 'Big Three' each have 20 grand slam titles now, following Djokovic's hot streak, and the Serbian admits it is unusual for both superstars to be absent.

"I have not experienced too many big tournaments in the past 15 years without Roger and Rafa playing. So it's a little bit strange," Djokovic said. "I'm used to seeing at least one of them. But still, some of the best players in the world are here.

"The guys who are in the top six, seven in the world, they are the biggest competitors or candidates for winning a medal. But potentially I am meeting with them in the later rounds."

After winning Wimbledon for a sixth time, Djokovic surprised many by claiming he could skip the Olympics, but he was reeled in by the appeal of representing his country, perhaps for the final time in the Games.

"Without the key element of any sports events – the crowds, the fans, that energy – it's different, but it is still the Olympic Games," he said.

"I was in a dilemma for a little bit, but I decided to come, and I'm glad because there are many more things that are beautiful about the Olympic Games. So I will try to focus on those things."