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Floyd Mayweather is this year's highest paid athlete. Source: Twitter

Once upon a time, the best athletes in the world were everyday guys who sold insurance in the off-season to make ends meet. Several baseball hall-of-famers took breaks from their playing career to serve in World War II, and Ted Williams was even shot down during one of his 39 missions as a fighter pilot in the Korean War.

Today, the modern media and advent of things like free agency have turned sports into a multi-billion dollar business and made even the benchwarmers millionaires. Some athletes like Tiger Woods and Michael Jordan have amassed billions with the help of endorsements. Big name athletes are among the most famous and respected celebrities, and the top earners bring in tens of millions of dollars in sponsorships each year. Below are little known facts about the world's highest paid athletes.

1. The bloodiest sport is still the most lucrative
Floyd Mayweather and Manny Pacquiao's fight on May 2nd resulted in nine-digit windfall for each boxer, making them the No. 1 and 2 names on this year's list of highest paid athletes. The event had been years in the making as both fighters have been considered the best in boxing but had never met in the ring. Mayweather, the victor, reaped $285 million, while Pacquiao brought in $125 million. With endorsements, Mayweather is expected to earn $300 million this year, while Pacquiao will take home $160 million.

2. Country club competitors are the most desirable pitchmen
Though they may not always be the best known athletes in the world, tennis and golf players are seen as the most attractive spokesmen, bringing in the most endorsement money. Of the top eight players in endorsement earnings, six came from tennis or golf. Tennis legend Roger Federer led the way with $58 million in sponsorships from high-end brands like NetJets, Credit Suisse, Mercedes, and Rolex. The only athletes in the top eight not from golf or tennis were Lebron James and Kevin Durant, and a large portion of their endorsement money comes from Nike. Since golf and tennis attract wealthier audiences than other sports, their top players are the most highly coveted by big brands.

3. For women, tennis is the best road to riches
Of the top 100 athletes, only two women made the list, both of whom are tennis players. Maria Sharapova ranked #26 with $29.7 million in earnings, while rival Serena Williams came in at #47 with $24.6 million. Unlike men, women's team sports do not generally attract a large audience, leaving individual sports as the best road to fame. Tennis, where men and women generally have the same calendar of events, is the rare sport where the two sexes are given roughly equal levels of attention. Winnings are generally the same for both men and women in most major tournaments. That attention helps top players draw sponsorships from the likes of Nike, Samsung, and Tag Heuer as Sharapova has enjoyed, and both she and Williams made more from endorsements than on-the-court earnings last year. 

4. Football and baseball players earn it on the field
Though football and baseball players are plentiful among the world's top paid athletes, many of them make little in endorsements. As part of a team, they are at a disadvantage to individual athletes whose glory is entirely their own. They are also less prominent than soccer and basketball players, both of which cater more to stars and have international appeal.

Football and baseball, on the other hand, are most popular in the U.S., and even the best players in each sport spend as much time, if not more, on the bench than in the action. Football players have the added challenge of donning helmets, which makes their face less recognizable to the general public. Peyton Manning is football's top pitchman with $12 million in endorsements, while no baseball player made more in sponsorships than David Ortiz with just $4 million.

5. Good luck making a living as a track star
Track & Field may be the most eagerly anticipated sport in the Summer Olympics with the 100-yard dash seen as the marquee event. However, with little fan base for events outside the Olympics and no infrastructure to drive attention to it, there is virtually no prize money to be earned in track.

World record holder Usain Bolt was the only runner on the Top 100 list with $21 million in earnings but virtually all of that came from endorsements. Bolt made just $15,000 in competition earnings in 2014, in part because he was injured for much of the year, but even first place in events at the USA Track & Field Indoor Championships last year only won $5,000. 

Without the fan base of team sports or even individual sports like tennis, golf, or boxing, the earnings of athletes in prominent Olympic sports like track or swimming are bound to be limited.

Jeremy Bowman owns shares of Nike. The Motley Fool recommends Nike. The Motley Fool owns shares of Nike. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.