Croatia's tennis world number 15 Marin Cilic is alleged to have failed a drug test at a Munich tournament in April and the International Tennis Federation is reportedly probing the case.
Reports suggest Cilic failed the test only two weeks after passing a doping control at the Monte Carlo Masters.
More details are expected to emerge next week after the 24-year-old Croatian sent his explanation to the ITF.
A croation newspaper says Cilic's test was allegedly positive due to "incautious use of glucose."
Reports suggest Cilic found out about the positive result during Wimbledon, adding this was the reason the 24-year old withdrew from the match against France's Kenny de Schepper.
At the time it was reported that Cilic had injured his left knee.
News of Cilic's alleged positive test came out only two days after the ITF banned Serbia's Viktor Troicki for 18 months for flouting doping rules.
Troicki, 27, was found guilty of failing to provide a blood sample during a drugs test at the Monte Carlo Masters.
The Serbian, currently No.53 in the world, said he would appeal his ban.
