Juventus are eight points above Inter Milan and need four points to claim a ninth consecutive title.
Ronaldo opened the scoring with a penalty and partnered with Paulo Dybala to score his second goal.
Ciro Immobile scored a penalty to pull one back for fourth-placed Lazio.
Ronaldo and Immobile are joint top scorers in Serie A with 30 goals each. Portugal captain Ronaldo is bidding to become the first Juventus player to win the Serie A and golden boot since Alessandro del Piero in 2007-08.