Roger Federer says there are questions to be answered around world number one 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.

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 which the case was heard while others, such as Nick Kyrgios, believed Sinner should have been handed a ban.

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 America’s Today programme.

“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.”

Sinner has been able to put the controversy aside to reach the US Open quarter-finals, where he meets fifth-seed Daniil Medvedev on Wednesday.