Rudi Garcia’s men began the season with 10 straight wins but have now been held to a trio of draws in succession and sit one point behind Juventus.
Cagliari goalkeeper Vlada Avramov was chiefly responsible for ensuring Roma were unable to score on Monday evening, the 34-year-old pulling off a number of fine saves.
Victor Ibarbo's header appeared destined for goal, but Morgan De Sanctis sprung to his left to pull off a terrific reaction stop.
Roma responded impressively, with Gervinho heading firmly against Avramov’s right-hand post and Kevin Strootman forcing the goalkeeper into further action from close range.
Cagliari looked to have taken a shock lead in the 54th minute, but the linesman's flag cut short their celebrations after Sau had headed home.
The visitors retained an occasional attacking threat and De Sanctis had to be alert when pushing away Sebastian Eriksson's long-range effort.
A header from Adem Ljajic was helped towards goal by substitute Nicolas Burdisso but Cagliari's keeper got down well to tip the ball around the post.
