Atletico, whose Wanda Metropolitano stadium is the venue for this year's final, won the first leg 2-0 and took a conservative approach to Tuesday night's duel at Turin's Allianz Stadium, where the home side went for goal from the starting whistle.
Atleti did an admirable job of keeping their shape, yet the pressure continued to build and the breakthrough came in the 27th minute, when Ronaldo got his head to Federico Bernardeschi's cross and beat Oblak to cut the visitors' edge in half.
Cristiano threatened twice more before half-time: a bicycle kick and another header that both went wide and the teams went to the dressing rooms with the tie still in the balance.
Three minutes into the second half, Ronaldo's header off Joao Cancelo's cross drove Oblak back into the net. Though the keeper made the stop, enough of the ball crossed the line for the referee to signal goal.
The match then settled into an extended quiet stretch until the 80th minute, when Allegri took the bold step of sending in 19-year-old Moise Kean, the star of last week's Serie A win against Udinese.
Kean had a chance to score the winner, but failed.
With five minutes left in regulation, Correa brought down Bernardeschi in the box and the referee awarded a penalty.
Ronaldo, who dispatched Juve from the spot last season for Real Madrid, fooled Oblak and converted to set off delirium among the 40,000-plus in the stands at the Allianz.
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