Novak Djokovic makes his case to be ranked atop the greatest champions in tennis history at this year's US Open, where he seeks an unprecedented 21st men's Grand Slam singles crown.

The 34-year-old Serbian, who began playing tennis at age four after swinging a mini-racquet as a toddler, will also try and complete the first men's singles calendar-year Grand Slam since Australian Rod Laver in 1969.

Djokovic won the Australian and French Open and Wimbledon titles to match the record 20 men's Grand Slam crowns owned by Roger Federer and Rafael Nadal, who are both injured and absent from the New York fortnight.

Now top-ranked Djokovic can move atop the career Slam list for the first time and achieve a feat that hasn't been done in more than half a century.

"I believe I'm the best, otherwise I wouldn't be talking confidently about winning Slams and making history," Djokovic said. "But whether I'm the greatest of all time or not, I leave that debate to other people.

"It's very hard to compare tennis, say, from 50 years ago to today, but I'm extremely honored to definitely be part of the conversation."

Djokovic has won 20 titles and reached 30 finals in 65 Grand Slam starts. He has reached 28 finals in his past 42 Slam singles starters, playing for the championship in two of every three events and claiming 19 crowns in the span.

The "Big Three" of Federer, Nadal and Djokovic have won 60 of the past 72 Slams since Federer captured his first at Wimbledon in 2003.

But six of the 12 they didn't capture were taken at the US Open and 35-year-old Nadal, out with a left foot injury, and 40-year-old Federer, sidelined by a knee injury, won't hoist a trophy at Flushing Meadows.

While Djokovic has flourished at a time when Nadal and Federer dominated the sport, he has made his own mark, winning eight of the past 12 Slams and taking a record nine Australian Open crown.

"It's all coming together," Djokovic said after last month's Wimbledon win. "I feel like in the last couple of years for me, age is just a number. I've said that before. I don't feel that I'm old or anything like that.

"I feel like I'm probably the most complete that I've been as a player right now in my entire career."

Djokovic began his pro career in 2003 and reached his first Grand Slam final at the 2007 US Open, falling to Federer, before winning his first Slam title at the 2008 Australian Open.

In all, Djokovic has won nine Australian Opens, six Wimbledon crowns, two French Opens and the 2011, 2015 and 2018 US Opens.

After hoisting the Wimbledon trophy last month, Djokovic put his trek to 20 Slam titles in perspective.

"The journey I've been through has been very rewarding for every segment of my game and also my mental strength, the experience understanding how to cope with the pressure in the big moments, how to be a clutch player when it matters the most," Djokovic said.

"That's probably, if I have to pick one, the one that I would point out as the highlight of my improvement in my assets that I have in the last 15 years on the tour, just the ability to cope with pressure."

Known in his youth for impressions of such players as Rafael Nadal and Maria Sharapova, "Djoker" has held all four Slam titles at the same time in his career, completing a non-calendar Grand Slam by winning his first French Open in 2016.

That first title on the red clay of Roland Garros also completed a career Grand Slam.

Djokovic is going for a men's singles calendar-year Grand Slam feat only managed by Australian Rod Laver in 1962 and 1969 and American Don Budge in 1938.

Budge did it was part of a run of six consecutive Grand Slam triumphs and was the first to pull off the calendar year achievement in any discipline, man or woman, singles or doubles.

Laver's 1969 sweep was the only men's calendar year Slam to come in the Open Era, which began in 1968.