It's a relationship that doesn't quite add up. One opinionated commentator believes he knows why.

Serena Williams is two matches away from winning the U.S. Open and completing the calendar Grand Slam.

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The world's No. 1 women's tennis player beat sister Venus in three sets in the quarterfinals to run her streak of consecutive wins in Grand Slam matches to 33. With two more wins in New York, Williams will become the first player since Steffi Graf in 1988 to win all four Grand Slams in the same calendar year.

Williams also would tie Graf with 22 Grand Slam titles, just two behind all-time leader Margaret Court.

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But Serena makes about $10 million less in endorsements than five-time Grand Slam winner Maria Sharapova, whom Williams has beaten 17 consecutive times. ESPN's Stephen A. Smith says he knows why.

"I believe that she is appreciated. I believe that she is respected, but I'll be very, very clear and unapologetically saying this: If she were white it would be exponentially bigger and better," Smith said on ESPN2's First Take, as reported by Breibart Sports.

"In any category, when it comes to women's tennis, whether it be on the marketing side or beyond, and the fact that she is second to Maria Sharapova, the fact that people say the things that they have said about her, that they seem to go out of their way to bring attention to things other than the performance, it ain't just about race. It's about sex as well, meaning gender, OK? There's more sexism than it is anything else," Smith said.

When Tiger Woods was on top of the golf world, he was by far the top endorsement earner. So does Smith have a point?

To a point, perhaps.

That Sharapova makes almost twice as much money as Williams from a purely competitive standpoint makes zero sense.

Sharapova is viewed as one of the most beautiful female athletes in the world. Williams is viewed as one of the most athletic women in the world, but other tennis players have said they don't want to make their bodies as big as Williams' because they think it's un-feminine.

But Williams' past -- her introduction into the game with sister Venus and father Richard -- likely played a part as well. Richard was accused of fixing matches between the players, and neither Venus nor Serena were known as honest, straight-forward interviewees early in their career.

Also, while Sharapova was comfortable with her love life being public -- she dated men's player Grigor Dimitrov for nearly three years before the couple split in the summer -- Williams never has talked about her love life.

She refused to answer questions about rumors of a relationship with her coach, Patrick Mouratoglou, for almost two years, and now, she has not commented on a relationship with rapper Drake, despite two photos of the couple kissing.

A combination of Williams' preferred privacy and Smith's sexism claim could share responsibility for Williams' relative inability to earn the endorsement money she deserves, based on her athletic ability alone.

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