Maria Sharapova's memoir Unstoppable (out Sept. 12) is excerpted exclusively in the current issue of PEOPLE
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A five-time Grand Slam winner and 2012 Olympic silver medalist, Maria Sharapova was poised for success at the 2016 Summer Olympics in Rio when her dreams of gold disappeared.

In early 2016 she tested positive for meldonium (also called Mildronate), a supplement she’d taken for years that had just been added to the World Anti-Doping Agency’s banned substances list, and the tennis pro was suspended from her sport for 15 months.

“It’s probably the toughest thing an athlete can go through,” Sharapova, 30, says in the current issue of PEOPLE.

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There are still some who think she used meldonium to enhance her performance. (The drug is sold over the counter in Russia and Eastern Europe as a treatment for heart ailments but was unceremoniously added to the banned list in early 2016 because it may aid oxygen uptake and endurance.)

But in her memoir, Unstoppable, Sharapova maintains the drug was prescribed by a doctor and she would have stopped taking it if she’d been properly notified of the ban rather than becoming one of 60 people to test positive for the drug in the year after it was added to the banned list.

  • To read exclusive excerpts from Sharapova’s memoir, pick up the latest issue of PEOPLE — on newsstands Friday

But Sharapova is back: On July 31 she won her first U.S. match in almost two years.

“Winning is a nice stamp of approval but I really just enjoyed the process of being back at a tournament,” she says. “I gained a new appreciation for the routines before the match and after — the small things that are usually automatic without any thought.”

Her comeback hasn’t been completely smooth (she’s been sidelined by an arm injury after her recent win) but the Russian-born athlete is used to rising above adversity.

In Unstoppable, which is out Sept. 12, Maria holds nothing back as she shares her immigration story and hard path to stardom.

“It was difficult, but putting words to paper really helped me through some hard moments,” she says. “Though, I don’t know a lot of strong people who have had an easy past.”