Title-holders Inter Milan secured an easy 2-0 win on Sunday over Ascoli to remain joint leaders of Serie A with Palermo, who beat Sampdoria at home by the same score.
Third-place Roma trail on 20 points, having won 3-1 in a late game against guests Fiorentina as Daniele De Rossi and a brace from Brazilian Rodrigo Taddei canceled out a goal scored by Fiorentina's Tomas Ujfalusi in the first half.
Promoted Atalanta follow one point adrift, after a 2-0 upset of troubled 10-man AC Milan, who like Fiorentina, Lazio and Reggina, received point penalties as a result of a match-fixing scandal.
Atalanta took the lead from a searing Nicola Ventola drive on 50 minutes and sealed their win in injury time, as Andrea Soncin fired home off the bar.
Milan, who have now suffered a second straight defeat and trail the leaders by 17 points, played the last 15 minutes with 10 men, when an injury sidelined Gennaro Gattuso after Carlo Ancelotti had already fielded three substitutes.
Ascoli started confidently at the Giuseppe Meazza, using the offside trap to frustrate Inter forwards Zlatan Ibrahimovic and Hernan Crespo.
Constant possession paid off for Inter on 40 minutes as captain Javier Zanetti easily flicked home an assist from Ibrahimovic.
An own-goal by Mirko Cudini sealed Inter's win on 53 minutes as the defender stunned his keeper, botching a low cross from the just-fielded Luis Figo.
A chance for a consolation goal went begging as Julio Cesar deflected a spot-kick taken by Michele Fini.
In Palermo, the hosts clinched their fourth straight win at home to Sampdoria.
Captain Eugenio Corini made it 1-0 on 35 minutes with a slightly deflected free kick and Cristian Zaccardo secured the win on 70 minutes, finishing off after a scramble in the area.
Sampdoria defender Christian Terlizzi was dismissed with three minutes remaining.
"We played an evenly balanced first half, but we played better in the second half, and I think we deserved the win. We have the best attack [23 goals], but today we were clever also in defense," Palermo coach Francesco Guidolin said.