Centurion Ibrahimovic inspires Milan rout
The home team scored all their goals in the first half and then strolled through the second to go level on 24 points with Udinese, one behind leaders Juventus.
Nineteen-year-old Luc Castaignos claimed an 89th-minute winner to give lacklustre Inter Milan a lucky 1-0 triumph at Siena, his first league goal since his move from Feyenoord in the close season.
Inter's second successive win lifted Claudio Ranieri's injury-hit team to 12th with 14 points from 11 games as they continued their climb up the table following a terrible start to the campaign.
"We are aiming for the scudetto," a confident Ranieri told reporters. "We've given ourselves the target of winning everything, we're going well and we have to carry on."
Adrian Mutu grabbed both goals as lowly Cesena beat Genoa 2-0 to move off the bottom.
The Romanian converted a 70th-minute penalty and scored with a shot on the turn 10 minutes later as Cesena, who have nine points, climbed above Lecce.
Palermo maintained their 100 percent home record with a 2-0 victory over Fiorentina whose coach Delio Rossi had two spells in charge of the Sicilian team last season.
Goals by Fabrizio Miccoli and Josip Ilicic gave Palermo their sixth straight home win, taking them to 19 points in fifth place.
Juventus, unbeaten after 11 matches and with a game in hand on their rivals, won 1-0 on Saturday at Lazio who are fourth with 22 points.
Ibrahimovic rattled the woodwork after 54 seconds, an early warning of what was to come, before Thiago Silva opened the scoring in the eighth minute by side-footing home a low shot from just outside the penalty area.
Eight minutes later the volatile Swede reached his milestone with a delicate chip from the edge of the box.
Ibrahimovic totalled 23 goals in two seasons with Juventus, 57 in three campaigns with Inter and, before Sunday, 19 since joining Milan at the start of last season.
Brazilian Alexandre Pato added a third from close range before Ibrahimovic converted a penalty just before the break as Milan made it six wins and a draw in their last seven league games.
It was, however, a tame match apart from some crunching tackles by their Dutch midfielder Mark van Bommel.
Bitter rivals Inter were far less impressive but snatched a win when Castaignos snapped up Thiago Motta's incisive pass and placed his shot into the bottom corner.
Siena had enjoyed the better of the game and their frustration boiled over two minutes later when Franco Brienza was sent off, receiving a second yellow card for kicking and breaking the corner flag after a decision went against him.