"At no point had Arnau indicated, verbally or otherwise, that he consented to Tian touching his penis," it said in court papers.
"Nor did he ever indicate any desire for him to do so."
Arnau was so shocked by Tian's actions that "he completely froze" until the masseur "threw off his towel and told him to turn over".
"At that point, having shaken off his initial state of shock, Arnau abruptly rose from the massage table and demanded that Tian leave the room," according to court papers.
The rattled client immediately called the police, who arrived and arrested Tian, the suit says.
A receptionist at the Men's Spa told the New York Post Tian had been working there for more than a decade.
Arnau's lawyer said: "While people may laugh about the sexualisation of spas – happy endings or locker room orgies — the vast majority of people go to spas for healing and relaxation – not sex."