From Billie Jean to Serena: How women's tennis won

Serena Williams celebrates after a victory in her women's singles final match against Russia's Maria Sharapova in the 2015 Australian Open tennis tournament in Melbourne on January 31, 2015. William West/Getty Images

This week, Indian Wells tournament director Raymond Moore stepped down from the dual-tour event after making comments about his perception of women's tennis as inferior. He was swiftly rebuked by Serena Williams, the USTA and other professional female athletes.

But his comments, and the reactions of Novak Djokovic and the ATP tour itself questioning the equal-pay structure at Grand Slams, have again raised the idea that women somehow don't pull their own weight in the sport. Djokovic later apologized for his comments.

Women in tennis fought for equal prize money for almost 40 years until 2007, when Wimbledon became the last of the four Grand Slams to grant the same-size awards to men and women. While women still struggle for equal billing and have to combat the occasional undercurrent of resentment when discussing equal pay in dual-tour events, tennis has become the most lucrative sport for women.

And it isn't even close.

Women in tennis win more prize money, are more likely to land big-money endorsements and enjoy more media coverage than their female peers in golf, basketball and even soccer. The only two women on the Forbes list of the 100 highest-paid athletes last year were Maria Sharapova and Serena Williams.

And women's tennis has never been healthier. Prize money has more than doubled to $130 million since 2009. The WTA agreed to a lucrative new television deal worth half a billion dollars over the 10-year life of the contract, and all main-draw matches will be available on TV or stream online, according to the WTA. The ATP offered approximately $186 million in prize money last year, according to ESPN's Stats & Info.

As a comparison, the LPGA has roughly $63 million in prize money, making it the No. 2 sport for women. The PGA Tour offers $325.2 million. Last week, the PGA Tour and LPGA announced a new alliance, which could mean joint events in the future.

But equal prize money for men and women in tennis isn't simply altruism. TV ratings and ticket sales for women's Grand Slam finals in the U.S. -- especially in the age of Williams -- are often on par with those of their male counterparts. Sports Business Daily's Wimbledon records show that women's finals often pull ratings nearly even with the men, but can lag significantly depending on the matchup.

"Clearly the ability to have both the best female and male tennis players in the world is one of, if not the main reason for our sponsorship, viewership, attendance and all the other categorical successes," said USTA CEO and former WTA player Katrina Adams.

However, there's an alchemy to the success of women's tennis that may be hard to duplicate.

It started in 1970. Billie Jean King, who would later go on to defeat Bobby Riggs in "The Battle of the Sexes" in 1973, had the idea for a women's tour -- a pitch that neither the ATP nor the USTA wanted to back when King approached them. So she and eight other women, now known as the Original 9, each signed a $1 contract to compete in their own tour. A sponsorship from Phillip Morris (then Virginia Slims) and a television contract gave them some financial breathing room.

"Nothing was ever the same after that," King told espnW during an interview at Wimbledon last year. "And the men will never give us credit."

"We're very fortunate to have had the leadership of Billie Jean King," said Ilana Kloss , a former player and the CEO of World TeamTennis. "A lot of things came together. You had her, you had the passing of Title IX, you had King-Riggs, you had Phillip Morris. They're all important pieces, and sometimes the stars are aligned."

Former player and ESPN commentator Pam Shriver pointed to the way tennis events crop up during a calendar year, different from the World Cup.

"The four majors help hugely on the world stage," Shriver said. "You have Wimbledon every year -- not like World Cup or Olympics every four years. We have four huge events every year.

"The way we have structured our sport puts us in the headlines," she said. "The individual nature, you see the face, you get to know the personality."

The individuality of tennis has helped women's success, said David Schwab, a senior vice president at sports agency Octagon. The global nature of the sport appeals to international and luxury brands, and individual players can be signed without having to negotiate with an entire league, he said.

But also, in a tennis broadcast, there isn't a helmet or a crowd of other players on the screen. There are just the two players in the match in a recurring visual. For Schwab, who advises brands on use of celebrities and athletes, the amount of time each player appears on screen increases their appeal.

"When you watch a match for two hours, you're just staring at them, and that recognition matters in marketing," Schwab said. "A brand may have six seconds of someone flipping through a magazine or watching TV, and being able to recognize a person is important."

Men can draw larger crowds to their matches on open courts, particularly if those matches are epic five-setters, and attendance and the difference in sets at the Grand Slams are often the basis for the argument that women don't deserve equal prize money.

"We had some decades when the women were carrying the sport and some decades where the men were carrying the sport," Adams said, "and then some when they were the most dynamic duo in the world, and that has a lot to do with the amount of work put into it by these athletes."

Despite intermittent criticism, the success of women's tennis stands as a beacon to other women looking to bolster their own leagues. It also serves as a blueprint. King has often consulted with soccer and other leagues looking to boost interest.

Ronda Rousey, Diana Taurasi, Abby Wambach, Danica Patrick -- the number of women in sports who are household names has never been higher. Yet, for the most part, female athletes are often compensated at a fraction of what their male counterparts make.

Except when it comes to Williams. According to Forbes, she received $11.6 million in salary and winnings last year, compared to Roger Federer's $9 million and Andy Murray's $6.3 million. Djokovic received $17.2 million. (Federer had $58 million in endorsements, compared to Williams' $13 million, according to Forbes).

Williams is a large part of the WTA's success right now. Last year, when she was going for the calendar Grand Slam, the 2015 US Open women's final sold out before the men's. And her matches can easily draw more viewers than the men; her 2014 US Open final on CBS against Caroline Wozniacki on a Sunday averaged 4.5 million viewers, more than double what the match between Marin Cilic and Kei Nishikori drew on Monday.

Williams is an encouragement to many other women who play sports professionally, such as Tamika Catchings, who is embarking on her final year in the WNBA. Not only has Williams dominated her sport at an age when many athletes are moving on to the next phase, but she did it as a black woman in a predominantly white sport.

"[She's been] seriously amazing through the highs and the lows," Catchings said. "For her it's been amazing for us to see, not only as a woman, but as an African-American women -- to be able to look up to her who had her struggle and had a lot of things go wrong but in spite of that be successful."

In general, television ratings suggest that tennis fans tune in for individual players such as Rafael Nadal or Williams in a final. And because Grand Slam television broadcasts often include a women's and a men's match in early rounds, it can be challenging to assign one a value over the other.

But it's not just the Slam purses that make tennis one of the best sports for women. It's also the cumulative earning potential at events on the women's tour year-round.

Anne Keothavong reached a career-high rank of 48th in the world before she retired in 2013. The British player now provides commentary for BT Sport. Despite not breaking through to the top 10, Keothavong was able to play 12 years as a professional, earning $1.3 million in prize money. That kind of earning potential makes tennis a viable sport for midtier athletes.

In the U.S., women who play in the NWSL or WNBA are subject to salary caps, limiting their earning potential and keeping them far behind their male counterparts when it comes to pay.

"Obviously the first one through five are way ahead of everyone else, but compared to other sports you can't really complain too much," Keothavong said.