Udinese Antonio Di Natale shoot and score a free kick against Inter Milan
Inter, aiming for a sixth straight win since the Brazilian's arrival in the San Siro dugout, took the lead after quarter of an hour through Dejan Stankovic.
Udinese then stunned Inter with goals from Cristian Zapata and Antonio Di Natale to go into the break 2-1 up, with Maurizio Domizzi adding a third midway through the second half.
The result left Udinese treading water in eighth, although they made up ground to within one point of seventh-placed Palermo, while Inter remained fifth on 35 points, six adrift of leaders AC Milan.
A sixth win here would have seen Inter climb to second and that looked on the cards when Thiago Motta pulled the ball back for Stankovic to fire past Udinese keeper Samir Handanovic in the 16th minute.
Inter though barely had time to savour their good fortune when the hosts were back on a level footing.
After a failed Inter clearance from a corner the ball fell to Zapata on the edge of the box with the Colombian defender shooting into the top corner of Luca Castelazzi's net.
Four minutes later Udinese were in front.
And the man responsible was Serie A's top scorer Di Natale who took his tally to 15 with a superb freekick which dipped over the Inter wall and out of Castelazzi's reach.
Udinese, recording their first win over their illustrious visitors in 14 attempts, thwarted any hopes Inter may have had of leaving the Stadio Friuli with anything to show for their visit when Domizzi struck in the 69th minute.
Sunday's action continues with leaders AC Milan hosting strugglers Cesena, Juve at Sampdoria, third-placed Napoli in Bari, Bologna v Lazio and Fiorentina v Lecce.
On Saturday, Roma stayed in the title hunt with a 3-0 win over Cagliari pushing them up into second place. Palermo beat Brescia 1-0 to keep alive their Champions League qualification hopes, and Parma beat Catania 2-0 to move up into midtable safety.