Croatian player Marin Cilic was handed a nine-month suspension by the International Tennis Federation on Monday after testing positive for a banned substance.
The suspension was back-dated to May 1 and will last through January 31, 2014.
Cilic tested positive for nikethamide, a stimulant, at a tournament in Munich last spring. The ITF said it accepted the player's contention that he ingested the substance inadvertently in glucose tablets and was not trying to cheat.
As a result, the ITF gave him a reduced penalty rather than a ban of up to two years.
"Mr. Cilic asserted that the nikethamide, for which he did not hold a valid TUE (Therapeutic Use Exemption), had entered his system through his ingestion of Coramine glucose tablets that had been purchased on his behalf from a pharmacy,'' the ITF said in a statement.
"The Independent Tribunal found that Mr Cilic ingested the nikethamide inadvertently as a result of taking the Coramine glucose tablets, and did not intend to enhance his performance in doing so.''
The 24-year-old Cilic, who reached a career-best ranking of No. 9 in 2010, has not played since pulling out from Wimbledon in June before a second-round match, citing a left knee injury. He has since missed the US Open and will also be ineligible for the Australian Open in January.
The ITF said Cilic will lose all his results, ranking points and prize money since the Munich tournament.
His best result since Munich was a runner-up finish at the Queen's Club grass-court tournament, where he lost to Andy Murray.
Cilic said in a statement he will appeal to the Court of Arbitration for Sport in Lausanne, Switzerland.
He said the banned substance came from a glucose tablet purchased at a pharmacy in France.
"Unbeknownst to me, the glucose tablets contained a substance that is banned in-competition (although it is allowed out-of-competition),'' he said.
"I wish to emphasise that I have never knowingly or deliberately taken any banned substances in my life and that I am opposed to any use of performance-enhancing substances in sport,'' Cilic added.
Cilic said he isn't able to comment further until the legal process is completed.