SERENA Williams has spoken out about being drug tested before — insinuating they’re not ‘random’ — and she’s just launched another scathing attack at the process.

In a series of tweets fired on Wednesday, the tennis legend questions whether she’s being discriminated against, while still choosing to take part in the tests.

The attack comes after Williams claimed she was treated unfairly in June, following a series of drug tests.

The 23-time Grand Slam champion unhappy at excessive targeting from US Anti-Doping Agency drug testers, after a tester from the group refused to leave her home in Florida.

Williams’ concerns come just as she was given a wildcard to compete in next month’s Rogers Cup in Montreal, tournament organisers said on Tuesday.

The 36-year-old joins a star-studded field which includes the WTA’s top 25 ranked players, which includes Australian duo Ashleigh Barty and Daria Gavrilova.

The three-times Rogers Cup champion Williams, 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.

She last played the event 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 wildcard 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 world No.1.