Miroslav Klose's late goal handed the Romans a deserved 1-0 victory.
Edy Reja, put in charge at the weekend following the sacking of Vladimir Petkovic, enjoyed a successful start in his second spell at the helm of Lazio.
Inter fell eight points behind Napoli, who are third and hold the last Champions League qualifying spot.
Earlier in the day, Napoli had closed the gap on second-placed Roma to just two points with a 2-0 home triumph over Sampdoria.
Dries Mertens scored twice for Napoli as they claimed their 12th win of the campaign.
Sinisa Mihajlovic tasted his first defeat since taking over Sampdoria in November.
Fifth-place Hellas Verona began 2014 as they ended 2013, with another impressive win.
Andrea Mandorlini's side beat Lazio 4-1 in their last match before the winter break and they were 3-1 victors at Udinese.
Luca Toni put Hellas on their way with two early goals before Roberto Pereyra pegged one back before the break. Juan Iturbe put the result beyond doubt by knocking in Hellas' third 20 minutes from time.
Kaka notched his 100th goal for AC Milan in a 3-0 win over Atalanta at the San Siro.
The Brazilian scored twice, his first allowing him to reach the landmark.
Bryan Cristante struck the third as the Italian giants secured only their fifth victory of the campaign.
Genoa ended an almost two-month wait for a win in Serie A by beating Sassuolo 2-0.
Alberto Gilardino scored the opener from the penalty spot before Alberto Bertolacci made it 2-0 before the interval.
Gilardino then saw another penalty saved by Gianluca Pegolo.
Torino's three-match winning streak came to an end in a 3-1 defeat at Parma.
The hosts' Marco Marchionni cancelled out Ciro Immobile's opener in the 34th minute before goals from Alessandro Lucarelli and Amauri wrapped up the victory for the Parma.
Serie A's bottom club Catania stopped the rot with a 2-0 victory over Bologna.
The victory was their first since November 9.
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