PARIS — Marin Cilic lost to Juan Martín del Potro in the second round of the ATP Masters event in Paris on Wednesday, but his overall feeling was one of relief.

Cilic, a former top-10 player from Croatia, was happy to be in action after being among the highest-ranked tennis players to serve a doping suspension. He compared his sensations on court to “a kid playing for the first time,” and said his positive test caused “the worst time of my life.”

In early May, Cilic tested positive for the prohibited substance nikethamide, an ingredient in glucose tablets he had taken. Cilic began taking glucose powder in 2011 to aid in the digestion and absorption of creatine powder, a permitted supplement that boosts energy levels. But when his supply of glucose powder ran low at a tournament in Monte Carlo in April, he asked his mother to buy him some more at a pharmacy. She bought a different form of glucose than he normally used, he said, and the tablets, with warning labels written in French, contained the banned substance nikethamide.

In September he received a nine-month suspension from the International Tennis Federation, which said it believed Cilic did not attempt to use nikethamide as an illicit performance enhancer. Cilic appealed to the Court of Arbitration for Sport, which last week reduced his suspension to four months.