Maria Sharapova has been banned for two years by the International Tennis Federation for using a prohibited drug.
The Russian was provisionally banned in March after testing positive for meldonium at January’s Australian Open.
The heart disease drug, which 29-year-old Sharapova says she has been taking since 2006 for health issues, became a banned substance on 1 January 2016.
The five-time Grand Slam winner said she “cannot accept” the “unfairly harsh” ban – and will appeal.
Sharapova will challenge the suspension, which is backdated to 26 January 2016, at the Court of Arbitration for Sport (CAS)
In a statement, she said the tribunal concluded her offence was “unintentional” and that she had not tried to use a “performance-enhancing substance”.
But she claimed the ITF had asked the tribunal for a four-year ban, adding it “spent tremendous amounts of time and resources trying to prove I intentionally violated the anti-doping rules”.
The tribunal ruling said Sharapova tested positive for meldonium in an out-of competition test on 2 February, as well as in the aftermath of her Australian Open quarter-final defeat by Serena Williams on 26 January. It treated both results as a single anti-doping violation.
The London 2012 Olympic silver medallist added: “I have missed playing tennis and I have missed my amazing fans… your love and support has gotten me through these tough days.
“I intend to stand for what I believe is right and that’s why I will fight to be back on the tennis court as soon as possible.”
Sharapova, then 17, became the first Russian to win Wimbledon in 2004, added the US Open in 2006 and the Australian Open in 2008, before completing a career Grand Slam with the French Open title in 2012.
She won the French Open again in 2014 and was Forbes’ highest-earning female athlete in the world for 11 consecutive years, until Williams claimed top spot this year.
The World Anti-Doping Agency (Wada) said in April that scientists were unsure how long meldonium stayed in the system, and suggested athletes who tested positive before 1 March could avoid bans, provided they had stopped taking it before 1 January.
However, Sharapova had already admitted she continued taking the substance past that date, saying she was unaware it had been added to the banned list as she knew it by another name – mildronate.
In reaching its verdict, the ITF recognised Sharapova had not intentionally broken anti-doping rules, as she did not know that mildronate contained a banned substance from January of this year.
But the federation said the Russian was “the sole author of her own misfortune”, as she had “failed to take any steps to check whether continued use of the medicine was permissible”.
Former world number one Caroline Wozniacki described Sharapova’s case as a “sad situation”.
“Tennis has a really strong anti-drug policy in place and it helps the sport really keep clean,” the 25-year-old told BBC Sport.
“It’s always a sad situation when someone is getting banned or you have heard they have failed a drug test – not only for Maria but for tennis in general.
“The ITF is doing its best to make sure nobody tries to go that route of taking any enhancing drugs, it’s unfortunate for anyone who did that unintentionally as well.”
Meanwhile, Wada said it would “review the decision, including its reasoning” and decide whether to appeal.