Former world number one Lleyton Hewitt urged the ATP to sort out the sport following the exoneration of Britain's Greg Rusedski on a drugs offence.
The Australian said he had little faith in the ATP's anti-doping measures after a tribunal found the ruling body of men's tennis could have been responsible for Rusedski's positive nandrolone test.
"I'd like to think tennis is clean but I can't say 100 per cent," Hewitt said. "Sometimes you are just not sure. I know I'm 100 per cent clean, sometimes there have been guys who look stronger in the fifth set than they are in first. You have to wonder about that."
Rusedski, who admitted in January that he had tested positive in Indianapolis last July, was cleared of the doping offence by a three-man panel appointed by the ATP yesterday.
The Canadian-born 30-year-old had based his defence on the fact that seven other male players were exonerated last year after an independent inquiry ruled that they had taken contaminated supplements handed out by ATP trainers.
The ATP accepted last year that it may have unwittingly fed its players banned performance-enhancing substances but said it had stopped handing out the supplements in May, 2003.