Novak Djokovic expressed his frustration at being "kept in the dark" about world number one Jannik Sinner's doping case, saying in Brisbane on Sunday that it was not a good look for tennis.
Anti-doping authorities said in August Sinner twice tested positive in March for the anabolic androgenic steroid clostebol and was cleared of wrongdoing by an independent tribunal that accepted his explanation of unintentional contamination.
The 23-year-old Italian faces a potential ban of up to two years after the World Anti-Doping Agency appealed that decision at the Court of Arbitration for Sport, Reuters reported.
"It's not a good image and not a good look for our sport," Djokovic told reporters ahead of the Brisbane International.
"You don't want to see that. I believe that in the last 20-plus years that I've been playing on the professional tour that we've been one of the cleanest sports. I'll keep believing in that clean sport.
"I'm just questioning the way the system works, really, and why certain players are not treated the same as other players."
Sinner's was not the only recent high-profile case in the sport as world number two Iga Swiatek accepted a one-month ban that ended on Dec. 4 after a positive test for trimetazidine, which she said was due to contamination of her sleep medication.
The International Tennis Integrity Agency maintains that all doping cases are dealt with based on facts and evidence and not a player's name, ranking or nationality, but has not been able to fend off allegations of double standards.
"The issue is the inconsistency and the transparency. We've been kept in the dark with Jannik's case," Djokovic added.
"I'm not questioning whether he took the banned substance intentionally or not. I believe in a clean sport, I believe that the player will do everything possible to be playing fair.
"I've known Jannik since he was very young. He doesn't strike me as somebody who would do such a thing. But I've been really frustrated as (have) most of the other players to see that we've been kept in the dark for five months."
Outspoken Australian Nick Kyrgios said the cases involving Sinner and Swiatek were "disgusting" for the sport and slammed authorities over what he saw as lenient treatment.
