Italian tenor Andrea Bocelli, 41, was forced to cancel a sold-out performance in the amphitheater at the foot of the Acropolis in Athens yesterday because of an unspecified illness. More than 4,500 ticket-holders will receive refunds. Meanwhile, in Manhattan’s Central Park, the New York Philharmonic was forced to cancel its free outdoor concert Monday after Mayor Rudolph Giuliani ordered the park closed so it could be sprayed against mosquitoes carrying the deadly West Nile Virus.