The AC Milan forward hit the crossbar in the first half and squandered a few sitters before scoring his side's winning goal from the penalty spot in the 56th minute.
"It's never happened to me before," he told RAI Sport. "I was unlucky, but I kept trying to the end.
"It is becoming a good group and above all a great squad, as we manage to climb out of difficult situations.
"When [Petr] Cech parried that shot I thought he would get it if I fired low, so I tried to raise the finish, but I lifted it too much."
The 2-1 win at the Juventus Stadium in Turin saw Italy qualify for next year's finals.