Serena Williams upset over drug testing

Serena Williams has hit back at the frequency with which she is selected for anti-doping tests

Serena Williams has again hit out at the frequency with which she is selected for anti-doping tests.

The Wimbledon runner-up 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," she tweeted.

In a second message, the 36-year-old added: "But I'm ready to do whatever it takes to have a clean sport so bring it on. I'm excited."

Figures from the United States Anti-Doping Agency show Williams has been tested more than any other tennis player so far in 2018.

Individual figures for athletes tested through the Tennis Anti-Doping Program are published at the end of each year.

Williams addressed the issue at a press conference ahead of Wimbledon after it was revealed the American was unhappy when a tester refused to leave her house, having turned up for a random test outside of the 23-time grand slam champion's allotted hour.

"How is it I'm getting tested five times in June? It's only June, I've been tested five times," she said.

"Tennis has given me so much. It's such an amazing sport. I feel like equality, that's all I've been preaching. If that's testing everyone five times, let's do it. It's just about being equal and not centring one person out."