Marin Cilic has labelled the past few months a "nightmare" after returning to action at the BNP Paribas Open following a doping suspension.
Cilic served a four-month ban, reduced from nine months by the Court of Arbitration for Sport, after testing positive for the stimulant nikethamide at the BMW Open in Munich in May.
The Croatian's explanation for the presence in his system of the stimulant, which is banned in competition, was that he had inadvertently taken it in glucose tablets that had been purchased for him from a pharmacy.
"I would call what has happened to me a nightmare because your life is not any more in your hands and you are in the position where your career can completely turn around," Cilic said in The Times.
"I could have been out for two years and what would happen after that?
"That would leave a huge psychological scar, how would I even come back from that was the worst part, the complete stress I had.
"Also, just figuring that this thing happened - that I was told inaccurately about the substance in my system was a huge shock for me because all this is so sensitive.
"My life, my career is at stake and it might look as if it is just a job, but for me it is a completely different perspective. I knew I had not tried to cheat."
Cilic was handed a nine-month ban by the International Tennis Federation's Anti-Doping Tribunal last month, who accepted he had not intended to enhance his performance and reduced his suspension from the usual two years.
However, both Cilic and the ITF subsequently appealed the decision to CAS.
The 25-year-old thought he should not have had to serve a suspension and should only have received a warning, while the ITF argued the period of suspension should be increased.
A hearing was held in London on October 16 and CAS announced its decision last Friday, meaning Cilic, who has dropped from 12th in the world rankings to 47th, was free to return to competition in Paris this week.
He overcame Dutch qualifier Igor Sijsling in three sets to set up a second-round clash with Argentinian world number five Juan Martin del Potro on Wednesday.