Serie A champions Juventus beat visiting Cagliari 4-0 with a Cristiano Ronaldo hat-trick to go three points clear of second-placed Inter Milan.
The Milan side can draw level on points by winning a big match at Napoli later Monday.
Juventus for now sit top on 45 points from 18 games.
Ronaldo, now on 13 league goals, netted seven in the past five games.
“It was a good start to the year, but the important thing is the team,” Ronaldo said.
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“We created chances, and scoring is always important. Our game is getting better game by game. But we have to work and keep improving.”
At Turin’s Allianz Stadium, Juventus played a lacklustre first half, but Ronaldo pounced on a bad clearance to strike in the 49th minute.
He then scored a penalty kick in the 67th minute and closed the scoring in the 82nd minute off an assist by substitute Douglas Costa.
Gonzalo Higuain, a second half substitute, scored the game’s third goal one minute earlier with a slightly-deflected drive.
AC Milan brought on returning star Zlatan Ibrahimovic in the 55th minute as they were held 0-0 at home to Sampdoria.
The result meant they continue a dismal run in spite of the arrival of the 35-year-old Swedish star last week.
The Devils, on 22 points, lie 13 points off the UEFA Champions League zone as the domestic season reaches the halfway mark next weekend.
Sampdoria are two points above the drop zone.
“We wanted a win and failing to get it was disappointing,” Milan coach Stafano Pioli said.
“We played a lot of balls into their midfield but lacked the quality and the precision needed to win the game. We tried until the end, but did not find the right chance.
“In my opinion, (Ibrahimovic’s) impact was positive. He helped the team and was very active. Possibly the team are not used yet to his strong personality.”
Atalanta hammered Parma 5-0 with a Josip Ilicic brace, repeating the scoreline they inflicted on AC Milan 5-0 in the previous round.
The sparkling Bergamo side opened with a marvellous Alejandro Gomez drive under the bar and went into the break with two more goals from Remo Freuler and Robin Gosens.
Ilicic sealed victory after the break, taking the squad’s tally to a league best of 48 goals.
Earlier on Monday, Bologna youngster Riccardo Orsolini drilled in a stoppage-time free-kick to salvage a 1-1 home draw after Marco Benassi volleyed in a majestic first half opener for visiting Fiorentina.
La Viola hired their former midfielder Giuseppe Iachini during a two-week New Year break to replace coach Vincenzo Montella, who lost five and drew two of his last seven games.
In Sunday action, third-placed Lazio won 2-1 at Brescia, fourth-placed Roma lost 2-0 at home to Torino and Verona beat SPAL 2-0.
Genoa went third-last as their new coach Davide Nicola made a winning debut by edging Sassuolo 2-1.
Lecce versus Udinese and the Inter Milan game complete Match Day 18 later on Monday.