Serena Williams alleges 'discrimination' in how often she is tested for drugs

Serena Williams, a 23-time Grand Slam tennis champion, is alleging discrimination after being singled out for drug testing.

Williams, who reached the finals of Wimbledon this month, took to Twitter Tuesday, saying she's the tennis player "getting tested the most."

"And it’s that time of the day to get 'randomly' drug tested and only test Serena," she wrote. "Out of all the players it’s been proven I’m the one getting tested the most. Discrimination? I think so. At least I’ll be keeping the sport clean #StayPositive."

Williams, 36, said in another tweet that she will "do whatever it takes" to keep the sport of tennis clean.

Williams has been at odds with tennis' regulatory body over the last few months. She spoke out at Wimbledon about the number of times she was asked to give a drug test sample.

Williams responded to questions at Wimbledon about a Deadspin article that claimed she has been tested in 2018 more than twice as often as other top American female players on tour, despite never having tested positive for a banned substance.

"I do know that I'm always tested. I'm always getting tested, all the time," she said at Wimbledon. "I didn't realize it was such a discrepancy with me, as well as against the other players that they listed. It will be impossible for me not to feel some kind of way about that."

Williams claimed a doping control officer showed up to her Florida home 12 hours before a scheduled appointment in June. When she wasn't there, it was designated a missed test, according to Williams.

"I was like, 'Well I'm totally not in the area because my hour is actually a long time from now, so I'm completely so far away,'" she recalled to reporters at Wimbledon.

Williams said she is having ongoing conversations with the U.S. Anti-Doping Agency (USADA) on the matter, including the ratio of tests between her and other players on the tour.

"I've played over 20 years and have always been extremely honest, and that's one thing I take pride on," she said at the BNP Paribas Open in Indian Wells, California, in March. "Especially having a daughter and having a kid, I'm never going to be able to look my kid in the eye and say, 'Mom cheated,' or, 'Mom did something like that.'"

"That is totally irresponsible," Williams added. "If you're going to beat me, you're going to beat me and I'm going to congratulate you. But I'm never, ever going to take an advantage and try to have an advantage over someone else because that's not who I am, and that'll never be who I am."

Williams gave birth to her daughter, Olympia, 10 months ago. She played in her first major tournament since taking maternity leave at this year's French Open.

The USADA and the Women's Tennis Association (WTA) have not responded to ABC News' request for comment.

According to the USADA website, "Athletes are subject to testing 365 days a year and do not have 'off-seasons' or cutoff periods in which testing does not occur."

The agency says it uses a variety of factors to decide when, where and how often to test athletes. Those factors include history of doping in the sport, the physical demands of the sport, the training and competition calendar and doping analysis statistics.

"The term 'random' is not an accurate way to describe anti-doping testing in the United States," the website states. "As USADA uses various factors to strategically plan when and where we test athletes in order to achieve maximum success in deterrence."