ANTHONY Warlow came within a cough of missing the first night of The Phantom of the Opera.     But as the master singer seeks to shake off a virus, and recover in time for tomorrow night's performance, another star has been unmasked.     Understudy Simon Pryce replaced Warlow as the Phantom in yesterday's matinee and was set to step on stage last Saturday.     "It was so bizarre," Pryce said.  "We were being made up in separate dressing rooms.  Tag-team phantoms."     Warlow's health deteriorated last Friday when he missed an evening preview and Pryce covered the role.     "I was given about an hour's notice," Pryce said.     "Adrenalin got me through."     Doctors who saw Warlow, 45, on Saturday morning advised further rest and Pryce, 35, was warned he may be needed.     "It was touch and go all day," Phantom publicist Eileen O'Shea said.     "Finally, at about 7 o'clock, Anthony agreed to soldier on."     Associate director Arthur Masella told the first-night audience at the Princess Theatre that members of the cast, including Warlow, were battling the flu.     Pryce remained in costume throughout the show and shadowed Warlow backstage.     "I was doing all his moves and costume changes without actually going on," he said.     "Sometimes, in the wings, I saw Anthony doubled over, just trying to get the energy he needed to go back.  But he did.     "He was magnificent."     Pryce toured Asia with The Phantom and played the title role several times.     Will he be replaying the role tomorrow?     "Hard to know.  Anthony's pretty resilient," he said.     Should Pryce also fall ill, there is another understudy waiting by -- Roy Weissensteiner.