The ‘greatest of all time' debate has resurfaced following Novak Djokovic's three Gram slam wins throughout the course of 2021. As a result, the Serbian superstar is now even with tennis royalty Rafael Nadal and Roger Federer on 20 grand slams.

The prospect of one of these three reaching the elusive 21 grand slams has put many tennis fans into a frenzy about who is the greatest.

Other names such as Pete Sampras, Jimmy Connors and Rod Laver have also been thrown around over the years as being the greatest players ever. To see who really is the GOAT (greatest of all time), we think it is best to do a ranking system based on nine key markers. The criteria for this include the following:

1. Grand slams won

2. ATP Titles won

3. Records against other great players

4. Overall winning percentage

5. Ability to play on all surfaces

6. Highest Peak

7. Longevity

8. Eye Test

9. Quality of Opposition

To begin with, we will narrow it down to three: Rafael Nadal, Roger Federer and Novak Djokovic - in no particular order. This is nothing against Sampras, Connors or Laver, but they are not quite at the level of the aforementioned trio.

This is demonstrated by how Sampras and Laver only won 14 and 12 grand slams respectively, whilst Connors managed just eight. Sampras also has a strikingly obvious weakness which was clay, corroborated by how he never made a French Open final.

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Whilst it is hard to compare and contrast eras, Laver, Connors and Sampras were less dominant than the big three playing today, who are all up against each other in fierce battles.

Obviously, tennis now is far better than it ever has been since there is more advanced technology to help athletes improve, which is why it is based on who was the most dominant in their respective eras.

1. Grand Slams Won

Based on the first criteria of the number of grand slams each superstar won, all three players are dead even with 20. In all likelihood, Djokovic probably will overtake Federer and Nadal due to a number of factors, such as form and overall health, but for the purpose of this article we'll do it based on currency. This gives all three players three points to start off.

2. ATP Titles

Throughout their careers, Federer is on top with 103, Nadal is next on 89 and finally, Djokovic is on 86. So here Federer gets three points, Nadal two and Djokovic one.

3. Records against other players

Since records against the other great players are the easiest way to cross-check who is the best player, this will be rated out of six rather than three.

Based on the stats already mentioned earlier, this may surprise some people but Djokovic has the better record over both Federer (27–23) and Nadal (30–28) whilst Nadal has a significant edge over Federer (24–16).

Switzerland's Roger Federer (R) stands next to Rod Laver as he celebrates with the championship trophy during the awards ceremony after his victory against Spain's Rafael Nadal (L) in the men's singles final on day 14 of the Australian Open tennis tournament in Melbourne on January 29, 2017. WILLIAM WEST/AFP/Getty Images

While this may seem like conclusive evidence, it is also dependent on other factors, such as if they all played each other in their prime. For example, Federer peaked earlier than both Nadal and Djokovic and thus is more likely to have a worse record.

Nonetheless, it is still a good measure since you have to beat the best to be the best. So, we have Djokovic getting six, Nadal with four and Federer two.

4. Overall Winning Percentages

Moving onto overall winning percentages, Federer has recorded the worst winning percentage with 81.9%, whilst Novak edges out Nadal with 83.3% compared to Nadal's 83.1%.

However, since Novak and Rafa are both so close they will get three points each here whilst Roger gets one.

The dangerous thing about winning percentages is that they can make players like Federer, who isn't playing at a very high level anymore, seem worse than they actually are.

Still, since we are basing it off their whole careers, winning percentages must be taken into account.

5. Ability to play on all surfaces

Unsurprisingly, Nadal comes in last here since he was won 13 grand slams on clay, five on hard court and two on grass. He also has the worst winning percentage on hard court and grass with 78% on each.

It is then very close between Djokovic and Federer, where Djokovic has won 12 grand slams on hard court, five on grass and two on clay, and Federer has won 11 on hard court, eight on grass and one on clay.

Whilst Federer has a better winning percentage than Djokovic on grass (87.4%-84.1%) and hard court (84.3%-83.5%), his record on clay is much worse (75.9%-80.4%), accentuating a weakness on clay which is less apparent with Djokovic.

Serbia's Novak Djokovic kisses the winners trophy after beating South Africa's Kevin Anderson 6-2, 6-2, 7-6 in their men's singles final match on the thirteenth day of the 2018 Wimbledon Championships at The All England Lawn Tennis Club in Wimbledon, southwest London, on July 15, 2018. (Photo by Glyn KIRK / AFP) / RESTRICTED TO EDITORIAL USE (Photo credit should read GLYN KIRK/AFP/Getty Images)

This is very close but Djokovic gets the edge because he has two French Open titles and is an overall better clay-court player than Federer, as delineated by his 17 clay court titles compared to Federer's 11.

6. Highest Peak

To see who really is the GOAT, it is necessary to have a look at who the best player was at their best (highest peak). The way this will be ranked is by how many grand slams won in a short amount of time as well as taking into account my memory of these players during these times.

Federer's peak was the earliest of them all beginning from the start of 2004 through to the end of 2007 where he won 11 grand slams out of a possible 16.

Even more impressively, he was almost like a bridge between the eras of tennis, with one side including the likes of Agassi and Sampras and the other including legends Nadal and Djokovic.

During this time, Federer was basically unstoppable on hard court and grass and was tearing up the world with his classy play. His play through these years was probably the most aesthetic of anyone anywhere at any time.

Nadal's peak is probably the hardest to judge since the one time that he won three grand slams in a year (2010), his previous and following seasons he only won one grand slam.

This, in conjunction with the fact that he has never won two grand slams for two years in a row, gives Nadal one point. He simply wasn't as dominant as Federer and Djokovic during their primes.

Djokovic is an interesting one because it is almost as if he has had two peaks, ranging from the start of 2015 through to mid-2016 where he won five out of a possible six grand slams, and then again from Wimbledon in 2018 through to the present time where he has won eight out of 13 grand slams and looks destined to win more.

Switzerland's Roger Federer celebrates a point during his men's singles semi-final match against Serbia's Novak Djokovic on day eleven of the 2016 Australian Open tennis tournament in Melbourne on January 28, 2016. AFP PHOTO / SAEED KHAN-- IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE / AFP / SAEED KHAN (Photo credit should read SAEED KHAN/AFP/Getty Images)

Drawing from these stats, we are going to give Federer three points since he was the most dominant anyone has ever been during a four-year span, Djokovic two and Nadal one.

7. Longevity

Since Federer won 16 of his 20 grand slams from Wimbledon in 2003 through to the Australian Open in 2010, it is fair to say this six-and-a-half-year span is his prime.

Shifting to Nadal, he has won at least one grand slam from his first French Open title in 2005, apart from 2015 and 2016 which he missed with injury.

Lastly, Djokovic has won a grand slam every year since 2011 and there is no sign of stopping anytime soon.

Therefore, Nadal gets the edge and then Djokovic bags second since he has won a grand slam for 11 years straight compared to Federer who won in just eight straight.

8. Eye Test

This is almost completely subjective but Federer gets the nod here thanks to his incredible elegance on the court. His serve and volley game is far better than the others and his strokes are smooth as butter.

Next, we have Djokovic due to his ability to always stay in points with his absolutely incredible defence. He also has the best backhand of the lot.

NEW YORK, NY - SEPTEMBER 13: Novak Djokovic (R) of Serbia shakes hand with Roger Federer of Switzerland after their Men's Singles Final match on Day Fourteen of the 2015 US Open at the USTA Billie Jean King National Tennis Center on September 13, 2015 in the Flushing neighborhood of the Queens borough of New York City. Djokovic defeated Federer 6-4, 5-7, 6-4, 6-4. (Photo by Clive Brunskill/Getty Images)

Getting one point here is Nadal, mainly because his serve and volley game and backhand are the worst of the three and he seemingly doesn't exude as much class or grit as Federer or Djokovic. In saying this, he does pull out the most miraculous shots of them all.

9. Quality of Opposition

Since Novak had to play Federer, Nadal and even guys like Andy Murray and Stan Wawrinka in their prime, he gets three points here.

Then, two points will go to Nadal since he had to play Federer in his prime at the beginning of his career.

Meanwhile, Federer got to play against weaker players than Nadal and Djokovic when he won his titles, such as Agassi, Safin and players who didn't win many/any grand slam finals. Therefore, Federer will get one point here.

After all that investigation, time to add up the totals!

Overall Leaderboard:

1. Novak Djokovic- 24 points

2. Rafael Nadal- 20 points

3. Roger Federer- 19 points

There you have it - Based on the rating system, Novak Djokovic is the GOAT of tennis.