WADA announced on Saturday it is seeking a ban of one to two years for the US Open champion, but indicated it does not plan to back date an eventual suspension — which would mean that Sinner could keep his second grand slam title even if he is found guilty.
The announcement was made while Sinner was on court against Roman Safiullin at the China Open in Beijing.
"Obviously, I'm very disappointed and also surprised of this appeal, to be honest, because we had three hearings. All three hearings came out very positively for me," the 23-year-old Sinner said after beating Safiullin on Saturday.
"You know, I was not expecting it. I knew it couple of days ago, that they were going to appeal, that today it's going to go official, so... it's surprise," the Italian added.
"We always talk about the same thing. Maybe they just want to make sure that everything is in the right position. Yeah, I'm just surprised that they appealed."
Sinner tested positive twice for an anabolic steroid in March but was not banned in a decision by an independent tribunal announced by the International Tennis Integrity Agency last month because the ITIA determined he was not to blame.
Sinner's accepted explanation was that the banned performance-enhancer entered his system unintentionally through a massage from his physiotherapist, who had used a spray containing the steroid to treat his own cut finger.
WADA said it filed an appeal on Thursday to the Switzerland-based 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," WADA said in a statement.
"WADA is seeking a period of ineligibility of between one and two years. WADA is not seeking a disqualification of any results, save that which has already been imposed by the tribunal of first instance."
WADA suggested the rules were not followed correctly despite prosecutor Nicolas Zbinden for tennis's integrity body — which accepted Sinner's version of events — being a lawyer who regularly works on high-profile cases for the global watchdog, including the successful appeal against Russian figure skater Kamila Valieva.
An appeal verdict at CAS could come quickly — even within just a few months — if the parties agree to cooperate. At least that's how it worked in another high-profile doping case in tennis involving Maria Sharapova.
Still, the case likely won't be resolved before Sinner begins the defence of his Australian Open title in January. Sinner can continue playing while the appeal is being heard.
