Having won the first leg 3-0, Zinedine Zidane’s men found themselves in big trouble as they trailed at home in the second by an identical score.
The match appeared destined for extra time as 90 minutes passed, yet Mehdi Benatia’s clumsy challenge resulted in a spot kick for the hosts.
Gianluigi Buffon was dismissed for protesting against referee Michael Oliver’s decision , potentially ending his European career.
Ronaldo, who slammed the resultant kick into the top corner of the net, is puzzled why the visitors were so vehement in their remonstrations.
“I really don’t understand why they were protesting, but football is like that,” he said. “If they didn’t make the foul, Lucas would score the goal. We’re happy to be in the semi-finals.”
Meanwhile, the striker was cool over his long wait before striking the decisive blow to the Italians’ hopes.
“I had to reassure myself. I was decisive and thank God I scored,” he said.
Despite Madrid’s progress, it was a chastening experience, as the Ballon d’Or winner admitted.
“It’s a lesson for us,” he concluded.