Antonio Di Natale and Dusan Basta scored second-half goals for Udinese who have managed to maintain last season's impressive form despite parting company with two of their most important players.
Milan were involved in a see-saw battle at Bologna, where Marco Di Vaio gave the hosts the lead, Clarence Seedorf and Zlatan Ibrahimovic replied for the title-holders and Alessandro Diamanti made it 2-2.
Atalanta were held 1-1 at home by 10-man Catania and Cagliari played out a goalless draw with Parma.
After 14 games, Udinese head the table with 30 points, one more than unbeaten Juventus who visit AS Roma on Monday. Milan are third with 28 points, alongside Lazio who won 3-2 at Lecce on Saturday.
Udinese finished fourth last season but were not expected to do as well this term after forward Alexis Sanchez joined Barcelona and midfielder Gokhan Inler moved to Napoli.
Guidolin set their target as reaching 40 points and said that was still his aim two weeks ago.
"The table looks beautiful, let us see how long it lasts," he said after Sunday's game. "It's a magical moment and I hope we can make it last as long as possible."
Udinese took 67 minutes to break down Chievo, finally scoring when 34-year-old Di Natale, last season's Serie A top scorer, found the net with a looping header from Mauricio Isla's pass.
Basta added the second with a diving header and Alberto Paloschi reduced the arrears as Udinese maintained a 100 percent home record.
Milan fell behind in the 11th minute when Di Vaio broke clear of the defence and cleverly lobbed the ball over the advancing Marco Amelia from just outside the area.
Dutchman Seedorf curled in the equaliser from 25 metres five minutes later, in an equally impressive effort.
Milan survived a penalty appeal in first-half injury-time when Seedorf appeared to block Diamanti's free-kick with his arm and the visitors escaped again when Di Vaio's shot hit the post after the restart.
Their luck continued when Ibrahimovic won and converted a controversial penalty in the 71st minute for his ninth league goal of the season.
It ran out one minute later, however, when Diamanti's low, 25-metre shot slipped through Amelia's hands for the equaliser.
"It's always annoying to concede goals and it's especially annoying because of the way we did it. For the second goal in particular, we made mistakes as a team," said Milan coach Massimiliano Allegri.
"Today the referee made mistakes against both teams. I didn't say anything after the game against Fiorentina when we had four decisions go against us and I won't comment now either."
Nicola Legrottaglie gave Catania an 18th-minute lead at Atalanta, where the visitors had Nicolas Spolli dismissed in the 43rd minute for a second bookable offence.
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