Serena Williams says she is a victim of "discrimination" as she is the most drug tested American tennis player.
The 23-time Grand Slam singles champion made the claim in a tweet on Tuesday, after doping officials visited her.
The American had previously expressed frustration about the volume of her tests earlier this month.
"Out of all the players it's been proven I'm the one getting tested the most. Discrimination? I think so," said the 36-year-old.
An article published by website Deadspin in June revealed that Williams had been tested out of competition by the US Anti-Doping Agency (Usada) on five occasions in 2018.