That came after lengthy delay in which Gianluigi Buffon saw red – the veteran goalkeeper's first dismissal in his 117th, and final, outing in the competition.
Mario Mandzukic's first-half brace, both headed efforts, sparked life into a contest that looked over after the first leg in Turin.
And when Keylor Navas – who seemed unsettled without suspended captain Sergio Ramos in front of him – fumbled a Douglas Costa cross at the feet of Blaise Matuidi, Massimiliano Allegri's side drew level in the tie with half an hour of the 90 still to play.