Tennis legend John McEnroe has doubts over Maria Sharapova's claims that she was unaware that meldonium had been added to a list of banned substances.
Former world number one Sharapova sent shockwaves through tennis last week when she announced that she had failed a drugs test at this year's Australian Open.
The Russian said that she had been taking meldonium for the past 10 years for medical reasons and did not know that the World Anti-Doping Agency had prohibited its use in January.
But American McEnroe, a winner of seven grand slams, believes it is unlikely that nobody in her backroom team knew of the rule change.
"It would be hard to believe that no one in her camp, the 25 or 30 people that work for her or Maria herself, had no idea that this happened," he told the Tennis Channel.
However, McEnroe went on to say that his ejection from the 1990 Australian Open was because he was unaware of a rule change whereby the number of code violations that led to a disqualification was shortened.
"Nobody told me, so it is possible that Maria did not know that, though it's extremely doubtful," he added.
Sharapova's failed test has split the tennis world, with Andy Murray suggesting that taking a drug that has performance-enhancing qualities, even if legal, is wrong.
But McEnroe took a different stance, telling BBC Sport: "If a drug is legal? That is like a no-brainer. I mean, are you kidding?
"People have been looking since the beginning of time for an edge, and you're constantly looking for these things in any way, shape or form."