Croatia's Marin Cilic, currently world number 15, failed a drug test at the April Munich tournament and the International Tennis Federation (ITF) is probing the case, local media reported on Saturday.
Cilic failed the test only two weeks after passing a doping control at the Monte Carlo Masters, the Slobodna Dalmacija daily reported, quoting "reliable sources."
More details will emerge next week as the 24-year-old Croatian sent his explanation to the ITF, the daily said.
"There are no comments until we will be able to comment," the influential Jutarnji List daily quoted Cilic's manager Vincent Stavaux as saying.
Cilic's test was allegedly positive due to "incautious use of glucose," the daily reported.
Local media said that Cilic had found out about the positive results of the test during Wimbledon, adding that 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 withdrew from the match due to his left knee injury.
Reports on 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 at 53 in the world, said he would appeal his ban.