Top-ranked tennis player Jannik Sinner is “very confident” that he will avoid a doping ban after the World Anti-Doping Agency appealed a decision to clear him of wrongdoing following two positive drug tests.

The Montreal-based body known as WADA announced Saturday that it is seeking a ban of one to two years for the U.S. Open champion and has appealed to the Switzerland-based Court of Arbitration for Sport (CAS).

“I’m still surprised but I will collaborate like I did before,” Sinner said Friday, the day before his first match at the Shanghai Masters. “I had three hearings in which the three hearings went all my way, which you know, it was good. But now let’s see.

“But I’m very confident that it comes out very, very positively. I would be very, very surprised if it would be the opposite side.”

The appeal means that the case might not be resolved before Sinner begins the defence of his Australian Open title in January. Sinner can continue playing while the appeal is being heard.

In a news release Friday, CAS said it formally registered the WADA appeal and was working to confirm a panel of three judges. The court appoints a panel chair, WADA picks a judge and the respondents — Sinner and tennis authorities — collectively can choose one.

With no panel yet confirmed to take possible requests on fast-tracking the case, CAS said “it is not possible to indicate a time frame for the issuance of the decision.”

The 23-year-old Sinner learned of WADA’s decision at the start of the Chinese Open, where he went on to lose to Carlos Alcaraz in Wednesday’s final.

“It’s not in a situation where I feel comfortable in, that’s for sure, because I thought it was over. And now once again. So it’s not easy,” he said.

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 in August because the ITIA determined the Italian player 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.