The Portuguese blasted in a crisp shot from the edge of the area for a fourth goalscoring game in a row.
He then smashed in a first-time shot from Gonzalo Higuain’s through ball before Leonardo Bonucci headed home to make it 3-0 before the break.
Ronaldo almost grabbed his first Serie A hat-trick as he hit the post, before Ignacio Pussetto’s late consolation.
Juve keeper Gianluigi Buffon made his 646th Serie A appearance, one short of Paolo Maldini’s all-time record. It was Buffon’s 700th league appearance including Serie B and the French top flight.
Inter Milan will go back to the top if they avoid defeat at Fiorentina (19:45 GMT) .