Serena Williams of the United States and Maria Sharapova of Russia shake hands after Williams won their women's final during day 13 of the 2015 Australian Open at Melbourne Park on January 31, 2015 in Melbourne, Australia. (Photo by Hannah Peters/Getty Images)

Serena Williams has won 21 major tennis championships, an Olympic Gold medal, and is coming off of a year where she came within a hair of sweeping tennis’s Grand Slam.

She was named Sports Illustrated’s Sportsperson of the Year, beating out former UFC champion Ronda Rousey and NBA MVP Steph Curry. She has been the single most dominant force in sports – at one stretch winning 53 of 56 matches.

So why will it take a failed drug test for her to become sports’ highest grossing female athlete?

Maria Sharapova, the Russian tennis star who at one point was considered Williams’ chief rival, has spent 11 years in the No. 1 spot with Serena hot on her heels the last few years.

On the court, Sharapova is a former No. 1 player and five-time Grand Slam winner. But when it comes time for her to face Serena, you might as well fit her for a Philadelphia 76ers jersey.

Sharapova is a stunning 2-19 in her career against Serena, including 18 straight losses. Her last win against Serena came in 2004.

Despite all of this, Sharapova earned nearly $30 million last year in endorsements and on-court purses compared to Serena’s $24 million. That streak looks to be coming to an end announcing on Monday that she had tested positive for the recently banned substance meldonium during the Australian Open.

The announcement, in which Sharapova copped to taking the drug for 10 years, set off a string of her most high profile sponsors, including Nike, Swiss watchmaker TAG Heuer, and Porsche either suspending or terminating their partnerships.

The question still remains to how, despite being the most dominant female athlete on the planet, Serena Williams has never topped the women’s money list?

As much as you want to believe that being a dominant athlete and a winner translates to big endorsement dollars – with male athletes such as Michael Jordan, Tiger Woods, LeBron James, and Curry being prime examples – on the women’s side, image is still everything.

While Serena’s an inspiration to young women all over the world, it has only started to make an imprint on Madison Ave. in recent years. Williams told the New York Times Magazine last year that she’s good with where she is and won’t force a company to get behind her.

“If they want to market someone who is white and blond, that’s their choice,” she said. “I have a lot of partners (JPMorgan Chase, Wilson Sporting Goods, Pepsi and Nike) who are very happy to work with me.

“I can’t sit here and say I should be higher on the list because I have won more,” she added “I’m happy for [Sharapova], because she worked hard, too. There is enough at the table for everyone.”

Williams’ situation compares to another dominant tennis player who lagged behind in endorsements: Martina Navratilova.

Navratilova was one of the most dominant players of the 1970s and 80s, but lagged behind her rival Chris Evert in endorsements largely because companies were not embracing openly gay athletes back then.

There are companies, even if they won’t admit it in 2016, who are still reticent to strap their brand to a black woman, even one as dynamic and charismatic as Serena.

“Corporations ultimately hire product endorsers who they think will connect with their client demographics,” Forbes writer Patrick Rische said last summer while investigating the Serena/Sharapova dynamic. “Tennis – despite its efforts to engage new communities of participants – is still perceived to be a country-club sport, where the typical demographic both playing and watching avidly is affluent and white.

“But how much [Serena] will profit from continued success may be limited if indeed corporate America is less willing to invest in an athlete who – in their eyes – won’t move the needle with their clientele.”

Now understand, Serena is still eating out here. Along with her endorsements and winners’ money, she and her sister Venus own a small stake in the Miami Dolphins, so her status is not in question. Serena will continue on and have three chances to tie and possibly break Steffi Graf’s record for most Grand Slam wins this year.

As for Sharapova, her appearance in the Aussie Open is likely her last for a while as a drug suspension awaits.

Fittingly, Sharapova’s last Grand Slam appearance ended in a straight set loss…to Serena Williams.

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