The Nerazzurri had comfortably won 2-0 when the two sides met recently in Serie A and this looked like going the same way until Juve started to come back into the game.
Juve had the chance to go level on 25 minutes when Inter's Ashley Young pulled back Juan Cuadrado in the box and, following a look at the VAR monitor, Gianpaolo Calvarese awarded a penalty which Ronaldo smashed right down the middle.
Inter stepped it up in the second half with Alexis Sanchez and Christian Eriksen missing good chances, while Juve's 43-year-old goalkeeper Buffon pulled off an impressive save to deny Matteo Darmian as Juve held on.
However, Juve comfortably saw the game off as they look to win the trophy for the first time in three seasons, having been beaten finalists to Napoli last term.
Ronaldo looked less than happy when manager Andrea Pirlo denied him any potential hat-trick by substituting Juve's No. 7 on 77 minutes but, speaking after the match, the Bianconeri manager said: "He [Ronaldo] knows he is our point of reference. But even he needs a breather to stay at the top."
The two sides meet for the second leg at the Allianz Stadium on Feb. 9 with Juve bringing an away lead back to Turin.