Serie A leader Juventus was upset 2-1 at home by 10-man Sampdoria and two other title contenders also fell Sunday, as the big Italian clubs struggled to get back into full gear after the holiday break.
Meanwhile, Edinson Cavani was on form and scored a hat trick as third-place Napoli beat Roma 4-1 in the late match. With 16 goals this season, the Uruguay forward moved two past Stephen El Shaarawy of AC Milan to top the league scoring chart.
AC Milan also struggled before beating Siena 2-0 with two second-half goals.
Milan is seventh with 30 points.
In other matches, it was: Parma 2, Palermo 1; Chievo Verona 1, Atalanta 0; and Genoa 2, Bologna 0.
Juventus went all of last season unbeaten but has now lost four times this campaign — and twice at home.
Sebastian Giovinco gave Juventus the lead with a penalty in the 24th and it looked like it would be easy for the Turin power after Sampdoria defender Gaetano Berardi picked up his second yellow card in the 31st.
But Mauro Icardi then scored twice for the visitors.
In the 53rd, Icardi finished off a counterattack with a long, angled shot that goalkeeper Gianluigi Buffon should have done a better job at stopping.
In the 69th, Icardi scored again from a similar position, this time using a powerful rising shot that skimmed over Buffon’s outstretched hands.
Juve had a chance to equalize but forward Mirko Vucinic’s shot in the 73rd hit the crossbar and then bounced down just in front of the goal.
In Udine, Di Natale put the hosts ahead in the 63rd by finishing off a swift counterattack as Inter’s defense struggled to keep up.
Two minutes later, Inter defender Juan picked up his second yellow card, and Colombia forward Luis Muriel doubled the lead in the 75th with a similar score.
Di Natale added another in the 79th from the center of the area — his 12th of the season — as defenders Walter Samuel and Esteban Cambiasso again couldn’t intervene.
In Naples, Cavani put the hosts ahead four minutes in after taking a through ball from Goran Pandev and shooting in off the arm of Roma goalkeeper Mauro Goicoechea. It was a similar start to the second half, with Cavani again hitting the target in the 48th from the center of the area.
Cavani struck again with a header following a corner in the 70th, Pablo Osvaldo pulled one back for Roma two minutes later, and Christian Maggio restored Napoli’s three-goal lead in the 90th.