Up 3-0 from last week's first leg in Turin, Madrid conceded to Mario Mandzukic's header after 90 seconds, with the Juve forward making it 2-0 on the night with a similar header eight minutes before half-time.

One of the biggest shocks in European football history looked a real possibility when Blaise Matuidi took advantage of Keylor Navas' dropped cross to make it 3-3 on aggregate just after the hour mark.

And the greatest drama of all came in the third minute of added time, when referee Michael Oliver saw a push on Madrid substitute Lucas Vazquez by Juve defender Mehdi Banatia, with the English official sending off Buffon before Ronaldo hammered the spot kick into the net to send the defending champions through to the last four.

Speaking to Spanish TV station Antena 3 following the final whistle, the 33-year-old said that from his position on the field the foul had been clear, and followed similar rough treatment from Juve's defenders throughout the game.

"I don't understand why they protested the penalty," Ronaldo said. "If he's not fouled, Lucas scores. He is hit from behind. During the game, Benatia and the rest were constantly kicking us from behind. That is their game. But Madrid are happy and in the semifinals."

Ronaldo admitted that his heart had been racing as he waited for Juve's protests to end, and for substitute goalkeeper Wojciech Szczesny to get his gloves on and take his place between the posts.

"The minutes before taking the penalty felt eternal," he said. "My heart-rate went up, but I calmed myself as I knew it would be decisive. Thanks to God I scored and we went through."

Zinedine Zidane's side would have to learn a lesson from how they allowed Juventus to nearly pull off such a dramatic comeback, Ronaldo admitted.

"Anything can happen in football," he said. "We did not play well, we must admit it, and it should be a lesson for the future.

"It was a game with a lot of suffering and reminded us that in football nobody gifts you anything, and you have to fight until the end. Today Madrid could have scored more, but Buffon and the others did well. We deserved to win."