Diego Milito's penalty five minutes from time gave Inter Milan a deserved 1-0 home win over Fiorentina on Sunday and extended their Serie A lead to eight points.
Second-placed Juventus, who host Inter next Saturday, lost 2-0 at Cagliari despite dominating for long periods.
Argentina striker Milito converted Inter's spot kick after being brought down, having earlier missed an easy chance.
Champions Inter, chasing their fifth straight scudetto, thought they had gone ahead early in the second half when Walter Samuel's header was controversially ruled out for shirt tugging.
Alberto Gilardino hit the post in Fiorentina's only real chance and Inter forward Samuel Eto'o continued his shaky form by wasting a late one-on-one with visiting goalkeeper Sebastien Frey.
"I have just one duty -- to be honest. I don't need to win anything, not the championship, not the Champions League," Inter coach Jose Mourinho told Sky when asked if his side must now focus more on Europe with Serie A becoming a stroll.
Juve, who lost at Bordeaux in the Champions League in midweek, have struggled to find fluency for most of the season under rookie coach Ciro Ferrara but were slightly unfortunate in Sardinia.
Nene opened the scoring with a rocket from just outside the edge of the area after half an hour and after sustained Juve pressure, including a strong penalty appeal, Alessandro Matri broke at the other end to seal victory for Cagliari late on.
Promoted Parma moved up to joint fourth with Sampdoria, beaten 3-0 by sixth-placed Genoa in Saturday's derby, thanks to a 1-1 home draw with Napoli.
Cagliari are seventh with Bari, AS Roma and Fiorentina just a point further back after 14 games.
Bari, who came up last season and have rejected two recent takeover attempts, came from behind to win 2-1 at home to bottom club Siena while Roma continued to improve with their own 2-1 comeback victory at lowly Atalanta.
Lazio's disappointing season drags on after a 0-0 draw against Bologna in Rome with their former coach Delio Rossi losing in his first league game in charge of Palermo, beaten 1-0 at Chievo.