MELBOURNE—Novak Djokovic faced at least 72 hours holed up in a Melbourne hotel for immigration detainees after he was denied entry into Australia on Thursday amid a political firestorm over his medical exemption from COVID-19 vaccination requirements. The tennis star, who is chasing a record-breaking 21st Grand Slam at the Australian Open, remained in the country after his lawyers launched an appeal seeking to overturn the federal government decision. A court agreed not to deport him before a full hearing scheduled for Monday. The saga has led to an international row, with Serbia’s president claiming his nation’s most celebrated sportsman was being harassed. “They are keeping him as a prisoner,” Djokovic’s mother, Dijana, said at the family’s restaurant in Belgrade. “It’s not fair. It’s not human.” She said she spoke to the champion on Thursday, and he was struggling to fall asleep. “His accommodation [is] terrible. It’s just some small, …