"I woke up yesterday morning with an inbox, in full capacity of love and compassion," Sharapova said to open the letter, which was posted on her personal website.
Her letter went on to explain that while she typically loves mornings, "it is fair to say that this day was not average. Nothing came to mind at 6am except that I am determined to play tennis again and I hope I will have the chance to do so. I wish I didn't have to go through this, but I do - and I will."
Sharapova, 28, announced at a Monday news conference in Los Angeles that she had failed a doping test at the Australian Open in January for meldonium, which became a banned substance under the World Anti-Doping Agency code this year. Meldonium is a blood-flow-promoting heart medicine that was banned because it helps oxygen uptake and endurance.
Sharapova said she had taken meldonium for a decade following various health problems, including regular sicknesses, early signs of diabetes and "irregular" results from echocardiography exams.
Fellow WTA Tour member Agnieszka Radwanska called it a mistake, but said she could understand how it happened.
"I'm scared because I know every pill can have something [prohibited] in it," Radwanska said Wednesday during a player roundtable at Indian Wells. "So when I'm sick I'm just taking aspirin because I'm always afraid there's going to be something else in it [medication]."
"It is difficult to imagine that something like this can happen, but mistakes are there and everyone can make mistakes," Rafael Nadal said. "I want to believe for sure it was a mistake for Maria, that she didn't want to do it, but it is a result of negligence. But the rules are like this, and it's fair, and now she must pay for it."
Russian Foreign Minister Sergei Lavrov told Russia's Ren TV on Thursday that the drug never should have been banned.
Lavrov said it was "a very strange decision, according to expert opinion", echoing comments made by the drug's inventor.
"In recent days there has been no limit at all to comments from specialists, including the inventor of this substance,'' Lavrov said. "They clearly and professionally explain that it isn't doping at all but a normal method for supporting the body and its basic functions.''
Lawyer Howard Jacobs told ESPN on Thursday that he has agreed to work with Sharapova.
"Because Maria has courageously stepped forward, our work will focus on the circumstances that led her to take this drug," Jacobs told ESPN. "Most cases wind up being about inadvertent positives, and this definitely fits that category."
Jacobs also said Sharapova's position is she received the same form letter over the past 10 years, informing her that a new banned drugs list was available but not personalizing it in any way or telling her she should be aware of changes.
"If you get the same email for 10 years, you're going to stop checking it," Jacobs said.
In the aftermath of her announcement, three of Sharapova's major sponsors have backed off deals with her: sportswear giant Nike, Swiss watch brand Tag Heuer and German luxury car company Porsche.
In her letter, Sharapova mentioned being followed by paparazzi and not going online much, "except the odd search for a new antique coffee table." She said her friends made a collage of all the "beautiful messages" from fans, along with their hashtags #IStandWithMaria and #LetMariaPlay.
"In this moment, I am so proud to call you my fans," her letter said. "Within hours of my announcement, you showed me support and loyalty, which I could only expect to hear when someone would be at the top of their profession."
Sharapova, who has been provisionally suspended by the International Tennis Federation until the investigation is finalized, concluded her letter by saying she would "like to play again and [hopes] to have the chance to do so."
"The good thing is she has acknowledged what happened, and she's facing it," Garbine Muguruza said Wednesday. "I guess that's a good thing she is doing, and we'll see how it goes."
A five-time Grand Slam winner, Sharapova is currently the No. 7-ranked player in the world.
ESPN's Jim Caple and The Associated Press contributed to this report.