After her absence last year, tennis star Serena Williams has re-emerged on Forbes' annual ranking of the world's highest-paid athletes for 2019, bowing in at No. 63 out of 100.

But her return is noticeable for another reason: She's the only woman on the list.

The lack of representation comes at a time when the issue of gender pay inequity in professional sports has gained attention and female players are willing to sue to put a spotlight on the disparity.

Williams, 37, reappeared on the list after Forbes calculated her pay at $29.2 million, as well as her winnings at $4.2 million and off-the-court endorsement deals at $25 million. Her sponsorships include Gatorade, Wilson Sporting Goods and JPMorgan Chase. She was also the only woman on the list in 2017.

Download the NBC News app for breaking news

Topping Forbes' list this year is Argentine soccer star Lionel Messi, whose pay, winnings and endorsement earnings reached a total $254 million.

Four other tennis players made the ranking, including Switzerland's Roger Federer, who reached No. 5 and earned a total of $186.8 million.

In general, the top athletes represent major sports in the United States and internationally: soccer, football, basketball and baseball. Mike Trout, the Los Angeles Angels center fielder who earlier this year reportedly signed what appeared to be the largest contract in professional sports history worth $430 million over 12 years, was No. 17 on this year's Forbes ranking.

Bob Dorfman, a sports marketing expert at the San Francisco firm Baker Street Advertising, said because the viewership and ratings remain high in popular team sports dominated by male athletes, that's where the blockbuster deals and big money will continue to flow to.

"It's what people are watching, it's what people want to see on TV and it's what the TV stations are willing to pay to get the rights to," Dorfman said. "And as long as there's more popularity, more appeal and more cache for the men's leagues, then that's how it's always going to be, regrettably."

Williams did not land on last year's Forbes list of the highest-paid athletes after taking a yearlong break from competition after having a child with her husband, internet entrepreneur and Reddit co-founder Alexis Ohanian. She has 23 Grand Slam titles and is highly regarded as one of the greatest tennis players of all time.

Earlier this month, she ranked on another Forbes list — America's richest self-made women — becoming the first athlete on there.

Be the brand. Starting @ShopSerena and Serena Ventures are just a few steps. Honored to be the first athlete on @Forbes #SelfMadeWomen list. https://t.co/aaREjvZ1y5 pic.twitter.com/GlBJnJEbaM — Serena Williams (@serenawilliams) June 4, 2019

"For tennis, it's all about endorsements and less about prize money," Dorfman said. "Serena is obviously a great businesswoman, and she's extremely marketable."

While a larger reckoning for how female athletes are paid remains elusive, experts say, there are efforts to bring the discrepancies to the forefront.

In March, more than two dozen members of the U.S. women's national soccer team filed a gender discrimination lawsuit against the U.S. Soccer Federation. The athletes say "institutionalized gender discrimination" exists in their pay, medical treatment, travel arrangements and overall workload.

According to the lawsuit filed in federal court in Los Angeles, American female players were paid a combined $1.725 million in bonuses after winning the Women's World Cup in 2015. To compare, American men were given $5.375 million in bonuses for reaching the round of 16 in the 2014 World Cup.

The U.S. Soccer Federation has declined to comment on pending litigation.

Meanwhile, the American women are favored to again win the World Cup, which kicked off last week in France.

T. Bettina Cornwell, the head of the marketing department at the University of Oregon's business school, said that while the audiences for men's sports have been larger historically and command more from sponsors, fans have some power to help change how much athletes are worth.

"From a societal perspective, if you want to address disparities, then watch women's sport. A great starting place right now would be the Women’s World Cup soccer," Cornwell said. "Any discussion of this not being an exciting sport to watch is simply rubbish. Be the audience they need and deserve."