Long-time pacesetters Inter drew 1-1 at Palermo on Saturday, the sixth time they have failed to win in their last seven league games, but second-placed AC Milan wasted the chance to seize top spot with a 1-1 home draw with Napoli.
Milan, who travel to inconsistent Parma on Wednesday, are a point behind with nine games left while third-placed AS Roma are four adrift of Inter before their midweek trip to Bologna.
"This is an atypical championship for several reasons, one is that a team (Inter) have won the league several times in a row now," Milan coach Leonardo told reporters.
"Milan are back and have the chance to fight. Roma have produced a great string of results and like us are trying to battle right until the end, but the ball is always in Inter's court given their general strength."
Milan have made up eight points on Jose Mourinho's Inter since February but the Rossoneri shot themselves in the foot against Napoli.
Ronaldinho was again on good form and Parma's defence will not enjoy facing him, but the 30-year-old Brazilian is still a long way from his Barcelona best.
The fact that he still stands out in Serie A is seen by many pundits as another damning indictment of the recent standard of Italian football, even though Inter have broken their Champions League last-16 hoodoo.
Inter have looked tired in Serie A in recent weeks despite their large squad, which may again be reduced by one against Livorno with banished striker Mario Balotelli still at loggerheads with Mourinho after a disagreement over commitment.
Livorno sank to the bottom after a 3-0 defeat at relegation rivals Atalanta on Sunday so Mourinho will hope even a jaded Inter can pick up three points and boost their confidence for Saturday's league game at Roma.
