With Atlético packing their defence, Ronaldo made the crucial breakthrough in the 27th minute when he rose to head in Federico Bernardeschi's cross at the far post.
Ronaldo has picked up in Italy where he left in Spain since joining Juve from Real Madrid for £88million last summer - and the forward was at his world-class best with his triple against Atletico, which takes him to 24 goals in all competitions this season.More news: Third Trump summit with North Korea`s Kim is likely
If Atletico Madrid score in the second leg, Juventus will be required to score at least four goals and would need to win by at least a three-goal margin to advance.
Ronaldo to score two or more goals - was 4/1, now 9/2.
Atletico are in Turin defending a 2-0 aggregate lead.More news: Ban Ki-moon, Tony Blair grace Akash-Shloka's grand wedding
Juventus defender Giorgio Chiellini, making his 500th appearance for the Italian champions, had the ball in the net after four minutes following a goalmouth scramble but it was disallowed for foul by Ronaldo on goalkeeper Jan Oblak.
It gave the home fans, who were handed 30,000 flags to wave, hope that their side could produce one of the great Champions League fightbacks in recent times. Bernardeschi also had several chances to extend Juve's lead, notably with an audacious overhead kick in the 35th minute which flew narrowly over. Leonardo Spinazzola makes his debut at the back! He had another attempt with a deflected shot a few minutes later, but didn't come close to the Atletico goal again until a late header sailed over the crossbar.
Can Ronaldo steer Juventus to European glory?More news: Donald Trump says Democrats are 'anti-Israel', 'anti-Jewish'
While one ecstatic fan said: "CRISTIANO RONALDO".