Mobido Diakite struck in the 88th minute, heading in from Cristian Ledesma's free kick, to give Lazio a 1-0 win over the Sardinians, leaving them with 51 points from 29 games and a three-point cushion over Napoli and Udinese.
Challengers Napoli squandered a two-goal lead in a 2-2 draw at home to Catania.
Milan, 2-1 winners over Roma on Saturday, lead with 63 points, seven more than Juventus who are at home to Internazionale later on Sunday.
There was little change at the bottom with a flurry of draws keeping Lecce, Novara and Cesena as firm favourites for the chop.
Lecce, 18th in the 20-team table, have 27 points, five adrift of safety, after a goalless draw against Novara, who are three points and one place behind them.
Cesena are a further six adrift at the bottom after a 2-2 draw with 17th-placed Parma (32 points).
Siena, who drew 1-1 at Chievo, and Fiorentina, held 2-2 at Genoa after Rodrigo Palacio scored an 89th-minute equaliser for the hosts, are both on 33 points.
Atalanta did manage a 2-0 win at home to fellow mid-table side Bologna, Manolo Gabbiadini and Simone Tiribocchi on target for the hosts.
