Serena Williams: 'Drug test isn't random, it is discrimination'

The former world number one Williams launched another attack on the process on Twitter on Wednesday morning after claiming she had been unfairly targeted by US Anti-Doping Agency drug testers in June.

She was bewildered when a doping official turned up at her Florida home at 8:30am on June 14, and refused to leave despite being 12 hours outside the agreed time on her athlete whereabouts form.

[RELATED - Serena Williams sends doping official away from Florida home]

Clearly unhappy about the ongoing situation, Williams hit out against the procedures.

"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," tweeted the American.

"But I’m ready to do whatever it takes to have a clean sport so bring it on. I’m excited."

Williams has been tested five times this year while some of her fellow American players have completely escaped the scrutiny of the authorities.

During her run to the Wimbledon final on July 1, Williams described her testing by the USADA as 'shocking'.

"I had a conversation with the lead guy with USADA," she said.

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

But she did appear to see the farcical nature of her treatment when one Twitter user asked her if they were "stealing your dna for a future nation of tennis pros?"

"I literally thought of that!!!" replied Williams.

Williams, 36 and showing no signs of decline after reaching the Wimbledon final, has been given a wild card to compete in next month's Rogers Cup in Montreal ahead of the US Open.

The 23-times Grand Slam singles winner, who gave birth last September and lost in the Wimbledon final 10 days ago, has not competed in Montreal since her run to the semi-finals in 2014 and last played the Rogers Cup in Toronto the following year.

"After announcing our player list a few weeks ago, we were only missing Serena to complete our dream tournament," Rogers Cup tournament director Eugene Lapierre said in a statement.

"Serena has impressed everyone with the speed at which she was able to return to a high level of play. She has always been a fighter and she is proving it once again."

Williams returned to competition at Indian Wells in March, where she lost in the third round, but has since looked close to her best and is ranked 27th in the world.

The wild card given to Williams, who won the tournament in 2001, 2011 and 2013, is reserved for players who meet specific requirements, including previously having been ranked number one in the world.