Serena Williams complained on social media that it's "test Serena" time again, as she once more criticized the frequency with which she is selected for anti-doping tests.

The 23-time Grand Slam champion took to Twitter on Tuesday night after she received another visit from a tester.

...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

But I'm ready to do whatever it takes to have a clean sport so bring it on. I'm excited. — Serena Williams (@serenawilliams) July 25, 2018

Williams has complained before about being drug-tested more than other American tennis players this year.

"Just test everyone equally," Williams said on the eve of this year's Wimbledon, where she lost to Angelique Kerber in the final.

A report by Deadspin said Williams had been checked five times in 2018 by June, more than other U.S. women and men in the sport.

Williams returned to competition this season after missing more than a year because of pregnancy and maternity leave. She gave birth to a daughter last September.

As part of the U.S. Anti-Doping Agency's "intelligent" out-of-competition testing program, it is not unusual for top athletes -- those who have had sustained success in their sport -- to be tested more often than others.

Following her loss to Kerber, Williams said she had proven to herself that she could still compete to win Grand Slams. Her next Grand Slam title would tie her with Margaret Court for the most with 24. She already has the most major trophies in the Open era.