This past weekend, men’s tennis superstar Novak Djokovic completed the career Grand Slam by defeating Andy Murray in the French Open final, the eighth man in history to achieve that feat. Not only that, but Djokovic became the third player to achieve a non-calendar Grand Slam by winning the last four tennis majors, the first in almost 50 years.

With an injury cloud over Rafa Nadal and Father Time slowly taking the lead on Roger Federer, Djokovic has clearly established himself as the best player in the world.  Not only that, but Djokovic is sneakily becoming one of the greatest men’s tennis players of all time.

Consider the following:

** Djokovic is now fourth all-time in men’s singles Grand Slam titles at 12, only behind Federer (17), Pete Sampras (14), and Nadal (14).

** Djokovic is tied for second with 20 men’s singles Grand Slam finals appearances, tied with Rafa Nadal.

** Djokovic is tied with Australian great Roy Emerson for the most Australian Open titles with six.

** Djokovic is now in the Top 5 for most cumulative weeks ranked No. 1.

** Djokovic is seventh all-time with 65 men’s singles titles, only four behind Nadal and less than 10 out of the Top 5.

** Djokovic now leads head-to-head meetings with each of his top three rivals: Andy Murray, Roger Federer, and Rafa Nadal.  His last six Grand Slam titles have come thanks to victories over Murray or Federer in the final.

Clearly, Djokovic is at the top of his game, possibly on his way to greatest ever status and that is not hyperbole. The march through the record books for Djokovic could continue this summer at Wimbeldon and the Olympics, with the gold medal being the only significant title missing now from Djokovic’s resume.

So, why is it that Djokovic continues in his failure to break through to the mainstream sports audience in the United States? Djokovic has garnered praise from those who follow the sport closely… but not much beyond that. What does explains the disconnect?

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1. Djokovic doesn’t have a signature playing style

The sport of tennis is synonymous with style. Each top player in the past and present seems to have that one calling card which is immediately identifiable. Serena Williams has her unmatched power. Pete Sampras had a thunderous serve. Andre Agassi had the best return of serve ever seen in the game. Rafa Nadal’s quickness and speed were unmatched. Roger Federer’s forehand was called the “greatest shot in the game” by John McEnroe.

What is Djokovic’s defining trait? What makes him identifiable in the world of tennis?  Truth be told, there isn’t one facet of Djokovic’s game that stands out from the rest. Djokovic’s style can only be ruthless in its precision, his all-around game just being better than every other player. He’s truly a tennis-playing machine, but sometimes machines have a hard time being popular.

PARIS, FRANCE - JUNE 05: Champion Novak Djokovic of Serbia celebrates with the ball girls following his victory during the Men's Singles final match against Andy Murray of Great Britain on day fifteen of the 2016 French Open at Roland Garros on June 5, 2016 in Paris, France. (Photo by Clive Brunskill for Adidas/Getty Images)
PARIS, FRANCE – JUNE 05: Champion Novak Djokovic of Serbia celebrates with the ball girls following his victory during the Men’s Singles final match against Andy Murray of Great Britain on day fifteen of the 2016 French Open at Roland Garros on June 5, 2016 in Paris, France. (Photo by Clive Brunskill for Adidas/Getty Images)

2. American Men’s Tennis continues its slow downward slide

Obviously, Djokovic doesn’t need an American rival to share the Grand Slam stage with, because Federer and Nadal haven’t had that either during their careers. And this isn’t one of those jingoistic angles that Americans demand one of their own be front and center in every sport.

It is a matter of fact, though, that men’s tennis in the United States has been in such a decline that a new rock bottom seems to have been found every year. John Isner remains the top ranked American player, currently at No. 17 in the world rankings. However, Isner has only made one career Grand Slam quarterfinal. Yes, that’s quarterfinal. In fact, Isner is mostly remembered for participating in the longest tennis match ever, a 5th set 70-68 victory over Nicolas Mahut at Wimbledon in 2010.

Amazingly, an American hasn’t won a men’s singles Grand Slam championship since Andy Roddick in the 2003 U.S. Open. Supposedly, there is now a wave of young Americans coming, but promise isn’t enough to recover from almost 15 years of irrelevance. Go around major professional sports and there probably aren’t too many franchises filling up stadiums after 15 consecutive losing seasons. That’s the state of American men’s tennis.

It was one thing for tennis fans in the States to cling to the Federer-Nadal rivalry, but if Djokovic is all alone at the top of the sport, and American tennis continues its slow downward slide, what compelling narratives are left for fans to sink their teeth into?

3. Djokovic hasn’t had the help of endorsements to reach casual fans

Let’s face it, as much as Darren Rovell is given a rough time for focusing on matters such as this, endorsements for major athletes matter. And it is especially true for individual athletes in niche sports. Think of names like Serena Williams, Rory McIlroy, lesser names like Rickie Fowler, more famous names like Tiger Woods. All have received extra exposure to the general public through endorsements. Can you even name the brands associated with Djokovic? When was the last time you saw Djokovic on a commercial?  Maybe it has to do with his major endorsers, including Uniqlo and Peugeot, instead of Nike.

In fact, Adidas went all-in on Andy Murray to make him their No. 1 man in tennis, leaving Djokovic with only a shoe deal.

By all accounts, Djokovic is an engaging figure off the court, as seen by this video of his impersonations of other famous tennis players. But Djokovic just hasn’t been able to break through the glass ceiling with causal American fans. Maybe more Grand Slam victories and a record chase will change the perception, but for now it’s an uphill battle.