Ronaldo opened the scoring with a trademark drive, after Aaron Ramsey’s pass, before threading a through ball for Federico Chiesa to score.
Ronaldo’s second goal took his tally this season to 14 from only 11 league games.
Marvin Zeegelaar scored a stoppage-time consolation for Udinese before Paulo Dybala added a fourth for Juve.
Andrea Pirlo’s side moved 10 points behind league leaders AC Milan, with a game in hand, while Udinese dropped to 13th and are without a win in four games.