Novak Djokovic—currently No. 1 in the world tennis rankings—was upset at the hands of Jon Isner, the No. 1 American male tennis player, on Friday in the quarterfinals of the Western & Southern Open. After the defeat, it has become clear that Djokovic isn't deserving of such a distinction.

The world tennis rankings are not based simply off wins and losses, nor are they compiled by a council of former players, trainers or experts. Instead, the ATP (Association of Tennis Professionals) Rankings are decided on a points system.

This points system factors in the value of certain tournaments over others, while also giving players who finished in the top-30 of the previous year's rankings to count their best six results from the ATP World Tour 500 and various other events to their final rankings.

More info on the ATP Rankings can be found here.

Djokovic may be the mathematical No. 1 player in the world, but that doesn't mean he deserves the title. The Serb has quality wins over Andy Murray (Australian Open) and Rafael Nadal (Monte Carlo Rolex Nationals) but has a number of disappointing losses.

He lost to Juan Martin del Potro in the semifinals of the BNP Paribas Open, Tommy Haas in the fourth-round of the Sony Open, Grigor Dimitrov in the second-round of the Mutua Madrid Open, Rafael Nadal in the semifinals of both the French Open and Rogers Cup as well as Andy Murray in the Wimbledon final.

Even with the Australian Open victory, it really hasn't been all that great of a year for Djokovic. A win at the Western & Southern Open would have made him the first man in history to win all nine Masters events, but the Cincinnati tournament has proven itself a worthy adversary. Yet another loss has put his overall tournament record this season at 3-8. The world's No. 1 player should have a much more impressive record than that.

Now, this isn't a knock on Djokovic's game. He's right up there with Rafael Nadal, Andy Murray and Roger Federer as the most talented players in the sport, and one could argue that he's more talented than each of the aforementioned players (all of which have been No. 1, except for Murray, by the way).

Murray would likely be the popular choice to replace Djokovic at the top given his emotional Wimbledon victory and overall strong season (wins at Brisbane International and Sony Open), but Nadal is the guy that deserves this ranking more than anybody else.

His season has been superb, aside from the first-round loss to Steve Darcis at Wimbledon. He's won the French Open, Rogers Cup and nearly every other tournament he has entered in 2013, and is bound to be a favorite for the US Open.

In fact, Paul Newman of The Independent cites that Nadal is actually close to re-gaining the No. 1 World Ranking anyway. His victory at the Montreal Masters makes gives him a mathematical shot to overtake Djokovic by the end of the US Open.

Newman writes:

Although it is more likely to happen at the end of the year or after the Australian Open in January, Nadal could return to No. 1 in just a month’s time. For example Nadal would overhaul Djokovic if he won the US Open, which starts in 13 days’ time, and reached the semi-finals at this week’s Cincinnati Masters if the Serb did not go beyond the quarter-finals of either tournament.

Now that Djokovic has already lost in the quarterfinals in the Cincinnati Masters, Nadal becoming No. 1 seems like even more of a possibility.

Both Djokovic and Nadal are quality players and will most assuredly bring tennis fans quality matches within the coming months to dissect and discuss. No matter what happens, though, Djokovic's 2013 season simply hasn't been good enough for him to deserve the top ranking.

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