The Montreal-based WADA announced on Saturday it was seeking a ban of one to two years for the US Open champion and has appealed to the Switzerland-based Court of Arbitration for Sport.
"I'm still surprised but I will collaborate like I did before," Sinner said on Friday, the day before his first match at the Shanghai Masters.
"I had three hearings which all went my way, 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 the case might not be resolved before Sinner begins the defence of his Australian Open title in January, but the Italian can continue playing while the appeal is being heard.
In a news release on 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 to appeal 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," he said. "And, now, once again. So it's not easy."
Sinner tested positive twice for an anabolic steroid in March but was not banned because an independent tribunal of the International Tennis Integrity Agency determined in August he was not to blame.
Sinner's accepted explanation was the banned performance-enhancer entered his system unintentionally through a massage from his physiotherapist, who used a spray containing the steroid to treat his own cut finger.
