AC Milan crashed 3-1 at Palermo on Sunday to hand city rivals and reigning champions Inter a six-point lead at the top of the Serie A table following their 2-1 success over Napoli.
Milan remain third on goal difference behind Juventus.
Milan coach Carlo Ancelotti tried to look on the bright side despite the defeat.
“After three months of good results I wouldn’t say this defeat is welcome, but it is useful,” he said. “For a while we’ve lacked attention and intensity. Tonight we didn’t play with rhythm. Our players were subdued all over the pitch, we played to Palermo’s rhythm and failed to play our own game.”
Palermo dominated for an hour, but Milan had the better chances of the first half.
Home goalkeeper Marco Amelia saved a Ronaldinho penalty on 27 minutes after he had clattered into Pato.
Ronaldinho also hit the bar in first half stoppage time direct from a free-kick, but Palermo had enjoyed more pressure.
Five minutes into the second period Fabrizio Miccoli faked to shoot on the edge of the box, made space and then curled a right-footer around Christian Abbiati.
Then, just before the hour mark, Fabio Liverani chipped the ball into the area and Brazilian forward Edison Cavani rose above Paolo Maldini to head home.
Brazilian Simplicio was left completely unmarked in the box to head in the third 10 minutes from time from a Federico Balzaretti cross.
Ronaldinho scored a consolation from the spot seven minutes from time after he was brought down by Cesare Bovo.
At the San Siro, Inter coach Jose Mourinho blasted the club’s fans for jeering Brazilian full-back Maicon during the game against Napoli.
“We need the support of our fans. I don’t like it when a section of our fans in a match such as this start whistling a player for a misplaced pass,” he said. “Maicon played despite an injury and despite both myself and the medical staff advising him not to. He said: ‘There’s no-one who can stop me playing.’”
On paper, Napoli looked to be a tough test for Inter, but the champions seemed to be strolling away with the game midway through the first period as they opened up a 2-0 lead.
Napoli gifted their hosts a 16th-minute advantage after poor defending at a short corner which was played to Sulley Muntari who crossed for Colombian center-back Ivan Cordoba to volley home on the turn.
Inter’s second was pure class, though, as Julio Cruz’s back-heel presented Maicon with space to shoot inside the box. He scuffed his effort, but it arrived at Muntari’s feet on the edge of the six-yard box and the Ghana midfielder reacted quickly to flick the ball between his legs and past the helpless Gennaro Iezzo on 25 minutes.
Nine minutes before the break the visitors fashioned a brilliant goal of their own.
Ezequiel Lavezzi cut in from the right flank and played the ball into Uruguayan Marcelo Zalayeta who, with his back to goal, back-heeled the ball past a line of blue and black shirts.
Argentina forward Lavezzi had continued his run and found himself clean through before cleverly dinking the ball over Julio Cesar as the Brazilian goalkeeper dived at his feet.
It was a fifth straight league win for Inter, who responded well to the midweek Champions League defeat at home to Panathinaikos.
Napoli remain fourth as a host of teams failed to capitalize on their slip up.
One of those was Fiorentina, who went down 1-0 to resurgent AS Roma through a goal from captain Francesco Totti just before the hour mark.