Roger Federer says there are questions to be answered around world No 1 Jannik Sinner avoiding a suspension after failing two anti-doping tests earlier this year.
Sinner tested positive for a banned substance twice in March but was absolved of fault or negligence by the International Tennis Integrity Agency (ITIA).
An independent tribunal accepted Sinner's explanation that clostebol, an anabolic agent that can aid muscle growth, had accidentally entered his system via a product one of his team had used to treat a small wound.
The decision was met with criticism in the tennis world, with some players surprised at the speed at which the case was heard while others, such as Nick Kyrgios, believed Sinner should have been handed a ban.
Kyrgios, describing the situation as "ridiculous", posted on X: "Whether it was accidental or planned. You get tested twice with a banned (steroid) substance… you should be gone for 2 years. Your performance was enhanced."
Federer believes that Sinner did not do anything wrong, but expressed his surprise that the Italian was not suspended until the case was resolved.
"I think we all trust pretty much that Jannik didn't do anything," Federer said on NBC's Today show on Tuesday.
"But the inconsistency potentially that he didn't have to sit out while they weren't 100 per cent sure what was going on, I think that's the question here that needs to be answered
"But look, it is what it is, and we need to trust the process as well of anyone involved here.
"I understand it's a tricky situation. It's the nightmare of every athlete and team to have these allegations and these problems because we fill out these forms all day every day.
"And it lives with you. Every morning when you wake up, you think, 'Is somebody at the door coming to test me?'. So it's really difficult.
"I understand the frustration of, 'Has he been treated the same as others?'. And I think this is where it comes down to."
After the ruling from the ITIA, Sinner said he wanted to put the "challenging and deeply unfortunate period" behind him.
Speaking to Sky Sports during the US Open he said: "In my mind I know that I have done nothing wrong, which in this case helps a lot.
"Obviously you cannot control every single thing of the outcome of this case."
The 23-year-old has since been able to reach the quarter-finals at Flushing Meadows, where he meets fifth-seed Daniil Medvedev on Wednesday.
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