WADA's appeal highlights its objections to the investigation and conclusion of Sinner's case. According to WADA, previous findings of the International Tennis Integrity Agency (ITIA) about the case were not correct.
"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's announcement came as Sinner was playing at the China Open. The 23-year-old star said he was disappointed and confused by WADA's decision.
"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," Sinner said after beating Roman Safiullin on Saturday.
ITIA cleared Sinner after a six-month investigation into his doping incident at Indian Wells in March. According to ITIA, an assistant accidentally injected the banned substance clostebol into Sinner during treatment. The dose was determined to be so small that it could not have affected his performance in any way.
Sinner was only banned for a few days during the six-month investigation. This has sparked controversy in the tennis world. Previously, some players, such as Grand Slam champion Simona Halep, were banned during the investigation process until the final verdict.
The ITIA was proactive and public about Halep's case, while Sinner's situation was not made public until August despite failing his doping test in March.
"Does it have anything to do with him being number one in the world? Possibly," tennis legend Mats Wilander told Eurosport. "And if it's possible that it does, that makes it even worse.
Sinner will have to wait to find out if he will be banned, after CAS accepts and considers the appeal from WADA. He has been on fire this year, winning two Grand Slams at the Australian Open and the U.S. Open, and climbing to world number one. He will face Jiri Lehecka in the quarterfinals of the China Open on Monday.