Serena Williams is furious at the "random" drug tests in her sport -- claiming she's been singled out by officials and tested more than any other player ... and she's saying it's "discrimination."
The 36-year-old tennis superstar -- who just came in 2nd at Wimbledon -- went off Tuesday night on Twitter as she got ready to take another drug test.
"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?