AC Milan slipped five points behind Serie A leaders Juventus when they lost 1-0 to their former midfielder Roberto Donadoni's Livorno side on Sunday.
Juventus, who beat lowly Brescia 2-0, are on 47 points from 20 games with Udinese a distant third on 34 points after they were beaten 2-0 by Reggina.
Milan's second defeat of the season was the result of an aggressive, adventurous performance by Livorno, who had already given the champions a fright by holding them 2-2 at the San Siro.
The visitors were lucky not to give away a penalty when striker Cristiano Lucarelli went down under a challenge by goalkeeper Dida, while at the other end Kaka rifled a shot against the post.
In the 28th minute, however, a foul by Jaap Stam gave Livorno a free kick midway inside Milan's half.
Dida reacted quickly to palm away a header by Portuguese midfielder Vidigal, but was powerless to stop striker Corrado Colombo firing in the rebound.
Far from trying to protect their lead, Livorno pressed forward. Milan had chances to equalise, notably when Andriy Shevchenko scuffed his shot when one-on-one with the keeper, but it was Livorno that looked more threatening.
Two minutes after the interval Luca Vigiani failed to hit the target alone in front of goal, and Igor Protti later forced Dida down to his near post with a shot from the edge of the box.
Milan's misery was completed when Alessandro Nesta received a red card for a second bookable offence deep in stoppage time.
From the free kick Lucarelli blasted the ball against the bar to underline the hosts' supremacy.
"We played badly and Livorno deserved to win," said Milan coach Carlo Ancelotti.
"It's a shame the gap has grown
NEDVED INJURED
Juventus comfortably beat a Brescia side that had earned just two points in their last five games.
David Trezeguet put Juve ahead in the 12th minute when he stabbed home Zlatan Ibrahimovic's low cross at the far post.
Juve needed a superb diving save by goalie Gianluigi Buffon to keep out a shot by Andrea Caracciolo and preserve their lead, but a second goal two minutes before halftime sealed the win.
Ibrahimovic accelerated into the area and cut the ball across the box. Brescia defender Maurizio Domizzi, under pressure from Trezeguet, turned the ball into his own net.
The only cloud on a bright day for the home side was Pavel Nedved's 70th-minute exit with a suspected knee injury after going down under a heavy challenge by Domizzi.
Elsewhere, Reggina strikers Emiliano Bonazzoli and Marco Borriello struck at the end of each half to give Reggina victory over Udinese.
It was also a bad day for Lazio, who fell behind after just 22 seconds of their game against Sampdoria when sloppy defending allowed Vitali Kutusov the space to blast a shot past goalkeeper Angelo Peruzzi.
Three minutes later Francesco Flachi doubled the visitors' lead from the penalty spot after being clumsily upended by Peruzzi.
Lazio striker Tommaso Rocchi pulled one back just past the hour, but could not prevent the Rome-based side suffering their second defeat in a row.
Sampdoria's 2-1 win lifted them to fifth, one point behind Inter Milan, who drew with Chievo Verona on Saturday.
In a later match, Fiorentina were at home to AS Roma.