Rafael Nadal has won $86.3 million in prize money during his career - but he still trails his rivals

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Rafael Nadal could win up to $3.7 million in the 2017 US Open.

In the sport of tennis, rank doesn't always equal wealth.

Rafael Nadal, the Spanish tennis player currently ranked No. 1 in the world, has advanced to the semifinals of the US Open Tennis Championships this week in New York City. He'll play No. 28 Juan Martín Del Potro Friday night for a spot in the finals on Sunday.

Nadal holds 73 overall titles - including 14 Grand Slams, which award the most prize money. The 31-year-old has been playing professionally since 2001 and is a two-time winner of the US Open.

Though Nadal's dominance in the sport is indisputable, his total career earnings - about $86.3 million - are not a reflection of his current top-player status, at least when compared to his rivals.

The Association of Professional Tennis (ATP) is responsible for ranking players, measuring their performance in the Grand Slam, Masters, and a handful of other tournaments throughout the year. A player's placement in those tournaments adds up to ultimately determine their numbered world ranking.

Serbia's Novak Djokovic, who's ranked No. 5, is currently the highest-paid tennis player in the world, with a total of $109.8 million in career earnings. Roger Federer, the Swiss man who sits at No. 3, has won $107.7 million in prize money.

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Novak Djokovic shakes hands with Roger Federer after winning the semifinal match at the 2016 Australian Open.

Tournament purses are consistently increasing, and more wins in recent years for Djokovic, who's been playing professionally since 2003, has likely added up to a higher total sum. In fact, the Serbian earned a record-breaking amount of prize money in one season in 2015 when he pocketed $20 million, winning three of the four Grand Slams that year.

On-court payouts are just one factor contributing to the multi-million dollar net worths of the top tennis players. According to Forbes, Nadal has earned $26 million from endorsements like Nike, Babolat, and Kia Motors, so far in 2017.

The deals are even more lucrative for Federer, who earns nearly $60 million a year from partnerships with Wilson, Credit Suisse, and Nike, and luxury brands Mercedes, Rolex, Lindt, and Moet & Chandon. He also commands fees up to $2 million for exhibitions and events.

Djokovic isn't far behind. The Serbian has earned $37.6 million off-court this year, endorsing brands like Seiko watches and Adidas. Earlier in 2017, he traded in his signature Uniqlo apparel for a new deal with Lacoste.

Nadal could win as much as $3.7 million at this year's US Open, if he takes the top spot. The United States Tennis Association is handing out a total of $50.4 million - the largest purse in tennis history. Both the men's and women's championship title winners will earn $3.7 million on Saturday, while the runner-up will walk away with $1.825 million. Semifinalists pocket $920,000.