Jannik Sinner said he was "disappointed and surprised" on Saturday after the World Anti-Doping Agency (WADA) appealed the decision to clear him in his steroid case, seeking a ban of up to two years.
The Italian world number one twice tested positive for traces of the banned substance clostebol in March but was exonerated and allowed to carry on playing.
The 23-year-old went on to win the US Open earlier this month for his second major crown, weeks after the International Tennis Integrity Agency (ITIA) found he "bears no fault or negligence".
The ITIA accepted his explanation that the drug entered his system when his physiotherapist used a spray containing it to treat a cut, then provided massage and sports therapy to the player.
WADA on Thursday appealed the high-profile case to the Court of Arbitration for Sport.
"It is WADA's view that the finding of 'no fault or negligence' was not correct under the applicable rules," it said in a statement on Saturday.
WADA made the appeal announcement as Sinner was on court at the China Open, where the defending champion fought back to beat 69th-ranked Russian Roman Safiullin 3-6, 6-2, 6-3 in Beijing.
"WADA is seeking a period of ineligibility of between one and two years," it added.
Sinner has consistently denied knowingly doping and said after his win: "Obviously I'm very disappointed and also surprised.
"We had three hearings. All three hearings came out very positively for me."
Clostebol is an anabolic agent prohibited at all times by WADA.
Player scepticism
After winning the US Open, Sinner said that the controversy over his failed tests was still on his mind.
He defeated Taylor Fritz in straight sets to add the New York title to his Australia Open triumph in January.
"It was and it's still on my mind. It's not that it's gone, but when I'm on court, I try to focus about the game, I try to handle the situation the best possible way, communication with the team, in the practice courts," said the world number one.
"It was not easy, that's for sure, but I tried to stay focused, with which I guess I've done a great job."
The decision to clear the Italian was met with scepticism by some players in the locker room who suggested that Sinner had benefited from preferential treatment due to his status in the sport.
Australia's Nick Kyrgios said in a post on X that the lack of a suspension was "ridiculous".
Canadian player Denis Shapovalov said the decision proved that there are "different rules for different players" suggesting that Sinner was saved by his ranking.
On court in Beijing, Sinner fell behind against the "lucky loser" Safiullin on Saturday after also dropping a set in his opening victory over Chile's Nicolas Jarry.
The second set went with serve until the sixth game when Sinner finally broke to go up 4-2, celebrating with a subtle fist pump.
Sinner took control early on in the deciding set on the way to an ultimately comfortable victory to reach the last eight, where he faces Jiri Lehecka of the Czech Republic.
In-form Sinner has reached at least the quarter-final of every tournament he has played this year.
AFP