Martina Hingis has retired from tennis after revealing she tested positive for cocaine at this year's Wimbledon.
Hingis shows the strain as she confirms her retirement
The Swiss star denied ever taking drugs and said the accusations against her were "horrendous and monstrous".
"I have tested positive, but I have never taken drugs and I feel 100% innocent," said the 27-year-old former world number one in a statement.
"The reason I have come out with this is because I do not want to have a fight with anti-doping authorities."
Hingis, a five-time Grand Slam champion and former Wimbledon winner, lost in the third round at the All England Club to Laura Granville.
She then underwent a routine drugs test, carried out under the auspices of the International Tennis Federation (ITF).
Her statement continued: "When I was informed I had failed my 'A' test following my defeat at Wimbledon I was shocked and appalled.
"I have no desire to spend the next seven years fighting doping officials. I'm frustrated and angry, and accusations such as these don't provide me with the motivation to continue.
"Because of my age and my health problems, I have also decided to retire from professional tennis."
In a statement, WTA Tour chief executive Larry Scott said his organisation knew nothing about any drugs test failure by Hingis.
"We have not received any official information regarding the positive doping test result referred to by Martina Hingis, and as a result we are not in a position to comment on the matter," he said.
"However, it is important to remember that in the area of anti-doping, all players are presumed innocent until proven otherwise."
A spokesperson for the Swiss tennis association told BBC Sport: "Our association makes it very clear that drugs is not, in any way, part of our sport.
"It is sad to hear this news, but we have yet to have been informed of the details from the tests."
The ITF have also "been told nothing", a spokesman told BBC Sport.
There is only one more thing for me to do - to thank all of you for many years of goodwill, and also to assure you: I have never taken drugs
Under the policy guidelines of the organisation, a player's identity remains anonymous throughout the process, from the initial positive 'A' and 'B' tests up to the independent drugs tribunal, which decides upon guilt and any punishment.
An outside company - International Doping Tests and Management (IDTM) - conducts the tests and processes the results in its laboratories in Montreal, before informing the ITF ahead of an anti-doping tribunal.
Whether a player is punished, stripped of prize money earned subsequently of the tests, or banned depends on a wide range of issues, including which substance was taken, whether and to what degree it was performance-enhancing, and whether the substance was taken deliberately.
Hingis's announcement is similar to that made by Greg Rusedski in 2004, when he admitted he had tested positive for nandrolone. The Briton was later cleared and his identity would therefore have remained anonymous had he not publicly admitted to the positive test.
BBC Radio 5 Live tennis correspondent Jonathan Overend added: "Hingis, in her words, has decided to confront the issue head on."
The Swiss star first retired in 2003 because of persistent injuries before returning to top-flight tennis at the start of 2006 and winning three more titles.
She brought her season to a premature end last month because of hip problems and is currently ranked 19th in the world.
Hingis's admission and second retirement brings the final curtain down on a glittering career which first took off in 1997.
In that year, she won her first Grand Slam title at the Australian Open, aged 16 years, three months and 26 days, and was world number one by March.
In July, she became the youngest player in the open era to win a singles title at Wimbledon, and also claimed the US Open in September.
She defended her Australian Open crown in 1998 and 1999, but was hampered by a succession of injuries in subsequent seasons and quit the sport in 2003, vowing never to return.