LOS ANGELES - Five-time Grand Slam champion Maria Sharapova says her 15-month doping ban has only sharpened her passion for tennis.

The former world No 1 from Russia, preparing to play at Stanford next week in her first WTA tournament in the United States since her suspension ended, wrote on the Players' Tribune website of the roller-coaster of emotions she has experienced since returning to competition in April.

"Though these last two years have been tougher - so much tougher - than I ever could have anticipated ... my passion for the game never wavered," Sharapova wrote. "If anything, it's only grown sharper."

Stanford tournament officials gave the 30-year-old Sharapova a wildcard entry as she gets set to compete in the tournament for the first time six years.

Similar wildcards offered upon her comeback sparked criticism from some of her WTA peers, who felt she should have to work her way back from suspension without such benefits.

Sharapova tested positive for meldonium, a heart and blood-boosting drug, at the 2016 Australian Open. She said that she had taken it for several years and did not know it had been placed on the banned list at the beginning of 2016.

Sharapova was issued with a two-year suspension, but the Court of Arbitration for Sport reduced it on appeal.

"I'm aware of what many of my peers have said about me, and how critical of me some of them have been in the press," she wrote. "If you're a human being with a normal, beating heart, you know ... I don't think that sort of thing will ever fully be possible to ignore."

But Sharapova said she'd been bolstered by the loyalty of her fans, describing how touched she was by the "Welcome back, Maria" signs she saw upon her return to the tour at Stuttgart.

Although her comeback has already been disrupted by a hip-muscle injury, Sharapova said she was looking forward to tackling the US hardcourt season.

She warmed up with a World Team Tennis appearance in Orange County, California, this week, and is now looking forward to competing in Stanford and Toronto.

"I'm sure my dozens of critics will show up, and so will my thousands of fans," she wrote.

"But, ultimately, who knows? When it comes to tennis, good or bad, there's really only one thing that I know for certain.

"I've missed it ... and I'm very glad to be back."

Agence France-presse