Serena Williams drug tested twice weekly while ranked No. 454

May 22 (UPI) -- Despite her No. 454 world ranking, Serena Williams says she is still drug tested twice weekly.

Williams commented on the frequency of the tests Monday on Twitter.

"And ... just like that anti doping is here ... again ... second time this week," Williams wrote.

"Proud to participate to keep the sport clean. Even if they do test me at my current ranking of 454 in the world. Two times every week."

Williams -- who attended the British royal wedding with husband Alexis Ohanian last weekend -- is set to compete in the 2018 French Open. But she'll try for her 24th Grand Slam as an unseeded competitor. She is up one spot in the WTA Rankings since the Twitter post. Her sister, Venus Williams, sits at No. 9 in the world.

Serena Williams and Alexis Ohanian attend the wedding ceremony of Prince Harry of Wales and Ms. Meghan Markle in a service Saturday at St George's Chapel inside the grounds of Windsor Castle in Windsor, England. Pool photo by Lionel Hahn/UPI

Serena was ranked No. 1 in the world when she left the court for maternity leave in January 2017. Her ranking was protected during maternity leave, enabling her to enter the French Open. But seeding is based on current rank. The French Open will be Williams' first Grand Slam tournament appearance since giving birth to Alexis Olympia in September.

Williams played as an unseeded competitor at the 2018 BNP Paribas Open and at the 2018 Miami Open. She won her first match in 14 months when she beat Zarina Diyas 7-5, 6-3 in the first round of the BNP Paribas Open on March 8 in Indian Wells, Calif.

"[A win] would mean a lot to me," Williams told UPI on March 19. "But right now, I'm taking a little bit at a time. I feel like every week I'm getting better and every day I'm getting better. So a little bit at a time and eventually I'll get there."

The 2018 French Open kicked off on Monday at the Stade Roland-Garros in Paris, France.