The comeback Serena Williams has made to tennis from having a child last year is nothing short of extraordinary. Williams tweeted on Tuesday night that she was “randomly” drug tested again, and bemoaned the frequency with which she’s being tested due to “discrimination.” She concluded with the silver lining that “at least I’ll be keeping the sport clean.”

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, according to ESPN. 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.

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.

Williams returned to competition this season after missing more than a year as a result of pregnancy. She gave birth to a daughter last September.

After losing to Angelique Kerger in the Wimbledon Finals, Serena 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, and she already has the most major trophies in the Open era. Williams will have that opportunity for number 24 in September at the US Open.