The 23-time Grand Slam winner had previously described her treatment by the U.S. Anti-Doping Agency (USADA) as shocking and raised questions about the number of times she had been tested this year.

Williams expressed her frustration again on Tuesday after a visit from doping officials: "...and it's that time of the day to get 'randomly' drug tested and only test Serena," the 36-year-old wrote on Twitter.

...and it’s that time of the day to get “randomly” drug tested and only test Serena. 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

— Serena Williams (@serenawilliams) July 25, 2018

"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, who gave birth to daughter Olympia 10 months ago, is working her way back on tour and her remarkable run to the Wimbledon final ensured she returned to the world's top 30 in the WTA rankings this month.

She arrived at the tournament on the back of a report revealing her anger about an unannounced test in June, when the tester refused to leave her Florida house.

Williams said at the time she had already been tested five times in June, while her fellow players were tested once or not at all.

Athletes returning from a long absence are often the subject of repeated drug tests, because they are seen as part of a higher-risk category than athletes in the flow of regular competition.