Lucas Vazquez did well to get in round the back and find Ronaldo, who had time to take a touch and send Oblak the wrong way.
Despite Ronaldo's heroics-and yet another prolific season in front of goal-he has also put in a number of subpar performances this year, and some Madridistas have been quick to vent their frustrations.
Ronaldo now has his sights set on another final, having become the first man since Robert Lewandowski in 2012-13 to achieve complete domination of a semi-final clash.
"He has the instinct for goals, he's unique", said Zidane.
Their chances of overturning the deficit in next Wednesday's second leg are remote given the dominance of their hosts at the Bernabeu and given the fact they don't have a player of Ronaldo's class.
Ronaldo's second, in the 73 minute, after a ball played towards him by Karim Benzema, was a pure strike - slamming a bouncing ball into the top corner with ridiculous power. 'He knows that the games mount up over the season and over the years and that he has to rest from time to time, but I don't think that is the only reason.
"I am happy to have scored my 400th goal for Real Madrid", Ronaldo added, although official statistics have him on 399. "I ended up scoring the goals, but the team did a tremendous job".
He said after the match: "We will try and take the chance we have, no matter how small it is".
"I have watched Atletico boss Diego Simeone a lot this season and he looks like someone who is coming to an end at the club, that he can not take the team any further".
Ronaldo has 35 goals in 41 appearances this season and is fast closing in on 400 for Madrid.
Real Madrid in the first semi-final will not help bale and Pepe, Atletico is also a loss - Juanfran, giménez and Fernandez missing a game, and part Vrsaljko is at issue.
"Football is a marvellous game and anything can happen". We can celebrate it now but then we go on to the next game'. "We need to be focused from the first minute until the last and we need the fans to be behind us".
He has now scored more goals in the competition than Atletico ever have.