By winning the 2021 US Open, Novak Djokovic would both lock up the first calendar Grand Slam on the men’s tour in more than 50 years and become the first man to ever win 21 Slams in his career.

With his natural athleticism and defensive abilities, Djokovic was destined to become a top-flight tennis player. But he has become perhaps the greatest men’s player ever because of his ability to turn the few relative weaknesses in his game into outright strengths.

Here’s a look at three areas where Djokovic turned the tables.

The forehand

Nov. 14, 2009: Paris Masters semifinals

The 2009 season has been a tricky one for Djokovic. After winning the Australian Open and two Masters-level tournaments in 2008, he has yet to reach another Slam final. He’s still performing well in the Masters events, reaching four finals in eight 2009 events, but he lost all four — two to Rafael Nadal, one to Roger Federer, one to the rising Andy Murray — and took only one set in the process. He is still one of the world’s best players, but he appears to have hit his head on a Federer/Nadal ceiling.

The indoor courts in Paris, however, present the perfect climate for showing off his evolving game. Having outlasted big-hitting Robin Soderling in the quarterfinals, he wallops Nadal in the semis 6-2, 6-3. He had won only one of their previous six meetings, but according to Tennis Abstract’s match charting, he’s given credit for 18 forehand winners to Nadal’s three, committing only four forehand errors in the process. He outlasts a game Gael Monfils in a three-set final to win his first Masters event in a year and a half.

In 17 previous meetings with Nadal that were charted by Tennis Abstract, Nadal had hit 254 forehand winners to Djokovic’s 221, and with 27% fewer errors. Beginning with this match in Paris, however, Djokovic flipped that completely around. In their next 30 charted matches, Djokovic hit 506 forehand winners to Nadal’s 365, and with only 10% more errors.

Djokovic had long been known as one of the game’s best returners and most agile defenders. His backhand was rarely in question. The evolution of Djokovic’s forehand was perhaps most notable against Nadal, but the most famous forehand he ever struck came against another great.

The fitness

Sept. 10, 2011: US Open semifinals

After blowing match points in a five-set loss to Djokovic in the semifinals the year before, Federer, in pursuit of his 17th Slam title, jumps out to a two-set lead over the Serb, 7-6, 6-4.

Djokovic, seeking his third Slam title in an all-time great year, fights back as he is known to do. He takes the third and fourth sets with relative ease, 6-3, 6-2, but Federer breaks to go up 5-3 in the fifth. Serving for a spot in the finals against Nadal, he creates two match points with an ace and a pair of quality second serves. He serves wide to Djokovic’s forehand, and Djokovic does something we have probably all done at some point in our amateur careers: Assuming the match is pretty much over, he swings at the ball as hard as he possibly can.

It becomes known informally as the Return Heard ‘Round the World. Djokovic’s exaggerated forehand paints the line and stuns Federer, who nets a forehand on his second match point and eventually double faults on break point. Djokovic wins 17 of the last 21 points, takes the fifth set 7-5, and powers through Nadal in a four-set final.

Djokovic finished the 2011 campaign with a 70-6 record. In Slams and Masters-level events, he went 58-2 with eight titles. (He would one-up himself in this regard four years later, winning nine such titles and the ATP Tour Finals in 2015.)

Perhaps no match signified how Djokovic’s game was so brilliantly coming together more than that semifinal in New York. He demonstrated incredible fitness by winning three straight sets against a player who, at the time, was already considered by many to be the greatest to ever live.

Djokovic hit 22 forehand winners to just 16 unforced errors, further confirming a once-flaky stroke as one of the sport’s better weapons. And he out-served one of the game’s greatest servers, winning a higher percentage of service points than Federer (71% to 63%) and creating more than twice as many break points (12 to 5).

The year before that match against Federer, Djokovic began to learn how to clear the biggest hurdle in his path. He had by that point already accomplished quite a bit and, as evidenced by a pair of five-set wins at the 2010 US Open, he could hold his own in long matches. But his body was randomly betraying him.

He had suffered eight match retirements from 2005 to 2010, including four in Slams: the 2005 and 2006 French Open, 2007 Wimbledon and 2009 Australian Open. And in other matches, his body quit even if he didn’t; after taking two of the first three sets in the 2010 Australian Open quarterfinals against Jo-Wilfried Tsonga, for instance, he had broken down and lost 6-3, 6-1.

With help from a Serbian doctor watching his struggles on television, however, Djokovic found out that he was gluten intolerant. As he wrote in his book, “Serve to Win,” a significant change to his diet almost entirely eliminated these mid-match breakdowns and allowed him to take his fitness levels to a place few have seen.

He has suffered only five match retirements in the past 11 years, three in the past nine. And his performance in long matches has gone from excellent to nearly untouchable.

With his fitness totally in check, other developments in his game became even more noticeable.

The serve

July 12, 2015: Wimbledon finals

In front of a rapt audience almost unanimously rooting for his opponent, Djokovic has, over the course of almost three hours, slowly wrung all suspense out of his fourth Wimbledon finals appearance in five years. His opponent, the Wimbledon record champion Federer, had broken his serve midway through the first set — the only time he would do so — but Djokovic broke back to force a tiebreaker he won 7-1.

Federer saved five set points in a thrilling second-set tiebreaker and took the set 12-10. But that was just about the last reason the crowd had to cheer. Djokovic broke Federer early in the third set and cruised, saving the only break point Federer created; in the fourth, he broke early again and didn’t allow Federer a single break point.

On match point, he hit an inside-out forehand winner to break one last time. Final score: 7-6, 6-7, 6-4, 6-3.

For the match, Djokovic matched Federer’s 14 aces with 14 of his own, and while their first serves were equally effective (Federer won 75% of his first-serve points to Djokovic’s 74%), Djokovic constantly neutralized Federer’s attacking chances with a brilliant second serve. While Federer won 49% of his second-serve points — not a bad average against the best returner in the game — Djokovic won 60% of his.

At this point, you could easily make a case that he had not only matched Federer’s serving prowess, but surpassed it.

Since returning from his 2017 elbow issues, his serve hasn’t quite reestablished the same level of dominance. But he has still won 69% of his service points in 2021, seventh on the men’s tour. When you do that and win 43% of your return points (second to Diego Schwartzman by only 0.5 percentage points), your opponent has little hope.

The future

Today: US Open

Djokovic takes the court on Tuesday seven wins from his ultimate piece of history. His odds of finishing with the most career Slam titles are high regardless of what happens in New York over the next fortnight.

But if he were to not only win a full-season Grand Slam of sorts, but do so at age 34, in the third distinct peak of his storied career, that would be something to which nothing in men’s tennis history could compare.

Pulling it off won’t be easy, however. For starters, the level of competition will be high. He needed five sets to get past Stefanos Tsitsipas in the French Open finals — he twice fell behind by two sets in that eventually triumphant fortnight — and four to take down Matteo Berrettini at Wimbledon. Both players have seen success in hard-court Slams: Berrettini reached the US Open semis in 2019, while Tsitsipas has reached the Australian Open semis twice in the past three years.

Two other members of the ATP top five, Daniil Medvedev and Alexander Zverev, have reached US Open finals within the past two years as well; Zverev won gold on hard courts at the Tokyo Olympics, and Medvedev’s torrid recent form — he has won 36 of his past 40 hard-court matches and took the Toronto title earlier in August — has moved him to the top of Tennis Abstract’s hard-court Elo rankings.

Djokovic’s fitness levels and dogged mentality have kept him ahead of the pack in the best-of-five Slam environments, but the next generation remains in hot pursuit, and he might need his best tournament performance of the year to win it.

To make things even more interesting, it was indeed Zverev who prevented Djokovic from a potential Golden Slam, taking him down in three sets in the Olympic semifinals. The match featured breakdowns from Djokovic in all of his primary areas of evolution.

Djokovic struggled to close things out, winning nine of the first 12 games but only one of the last 11. His serve faltered — he was broken twice in the second set and three times in the third. He managed only seven forehand winners to Zverev’s 19. And then he lost his composure in a bronze-medal match against Pablo Carreno Busta as well.

This was everything Djokovic hasn’t been in a while. Was it a momentary glitch? A crack in the aura of invincibility? We’ll find out over the next two weeks.