World No. 1 Novak Djokovic has collected $6.3 million in prize money this year. If he wins his fourth U.S. Open on Sept. 12, he will add $2.5 million to his tally, as will the women’s champion.

But the millions and private jets associated with the sport’s elite don’t extend far down the rankings.

Somewhere around 150th in the world, according to estimates of a handful of tennis agents, former players and insiders, prize money barely covers expenses for pros who employ full-time coaches and hope to make a modest living.

For them, getting a chance to compete at the U.S. Open — or any of the sport’s Grand Slam events — represents a chance not only to boost their rankings but also earn significant money that can sustain their training.

At this year’s U.S. Open, first-round participants are guaranteed a record $75,000 payout. The challenge for lower-ranked players is getting a spot in the tournament.

The world’s top 104 are guaranteed a spot in the U.S. Open’s main draw based on their ranking. For others, there are two primary routes: Earn their spot by winning three qualifying matches or get granted one of the 16 main draw wild-card entries (eight for men, eight for women) bestowed by the U.S. Tennis Association.

Wild cards, which are widely coveted and little understood, function as a financial grant to promising players.

“Obviously, tennis is an extremely expensive sport: You’re traveling around the world year-long. You have to pay for your coaches, your travel. There are so many different expenses,” says Washington native Hailey Baptiste, 19, who earned a U.S. Open wild card for a second consecutive year based on her promising results.

“The prize money continues to grow for first round and even for [qualifying],” added the 181st-ranked Baptiste, who learned to play at Rock Creek Park’s tennis center before enrolling in the full-time training program at College Park’s Junior Tennis Champions Center. “It definitely takes a lot of pressure and weight off your shoulders when you get that first-round check. And it’s a big help for anybody who is outside the top 100 or 150 or so.”

Martin Blackman, the USTA’s general manager of player development, said the organization doesn’t comment on its process for awarding wild cards.

According to Ray Benton, a veteran tennis agent, tournament director and current CEO of the Junior Tennis Champions Center, U.S. Open wild cards function like any other corporate investment.

“You want to get the most potential return,” Benton said. “USTA High Performance is in the business of developing American players who would be in the top 10 or 20 in the world. The wild cards are assets they have, and they spend them where they think they can get the greatest potential return.”

Wild cards work much the same at the other three majors — Wimbledon and the Australian and French Opens — with the respective national governing bodies using them to invest in their promising young players, reward hard work and support continued development.

Unlike pro football and basketball players, tennis pros aren’t salaried. They operate more like miniature corporations — independent contractors, technically, that must balance the books each season, with revenue covering expenses and, ideally, a bit left over to live on.

It’s a nearly year-round grind traveling the world in pursuit of ranking points and prize money in tour-level events, if they can qualify, or in lower-paying Challenger or ITF-level events one rung below.

Heading into this year’s U.S. Open, wild card recipient Ernesto Escobedo, 25, had competed in 17 tournaments in nine countries scattered among four continents in the past eight months.

Of those 17 tournaments, 11 were Challenger events, the equivalent of Class AAA baseball, where the purses are small and the competition fierce.

“Everyone in the top 300 can play,” said the 178th ranked Escobedo, who has been as high as 67th and is working to get back inside the top 100. “Challengers are super tough because everybody there wants to win really, really badly.”

At some Challenger events, Escobedo says he earned less than $1,000.

His attempt to qualify for the season’s first three majors was more lucrative. Though he failed to make the main draw at Wimbledon, the Australian or French, he earned nearly $19,000 for each qualifying effort.

His strong showing at an ATP tour level event in Los Cabos, Mexico, where he upset fellow Americans Mackenzie McDonald and Denis Kudla, strengthened his case for a U.S. Open wild card, which guaranteed his fifth appearance in his favorite Grand Slam.

“I had no idea they were going to give me one; I had no clue!” said Escobedo, who was notified one week before the tournament got underway, along with the other wild cards. “It feels amazing, and I am so grateful.”

As helpful as a wild card is, Baptiste said she preferred to earn a main-draw spot through qualifying, as she did at this year’s French Open. Getting in as a qualifier builds confidence, she said, and puts a higher ranked first-round opponent on notice that they’re about to face an upstart who’s on a roll.

“But I wouldn’t turn down a main draw wild card!” Baptiste added.

She lost her first-round match to Shaui Zhang of China in straight sets but considers it another step forward — particularly after getting injured in a Wimbledon tuneup in Berlin.

Escobedo fared better, steamrolling his first-round opponent, Pablo Cuevas of Uruguay, 6-1, 6-3, 6-1, in 89 minutes to advance to second round.

In six years as a pro, it equals his best U.S. Open result. It also represents the biggest payday of his career, with players who reach the second round guaranteed $115,000.

That will more than double the $105,957 winnings Escobedo has earned all year for his 17-tournament grind.

But he plans to pour it all back into his training, paying his coach, Javier Nalbandian, and his trainer.

“It is expensive, but I’m going to do it the correct way,” Escobedo said. “If I have a good coach, a good team around me, it gives me confidence. I feel like there is no limit for me.”

Read more: