Germany forward Mario Gomez scored for the second match running to help Fiorentina beat Chievo 3-1 and AC Milan's crisis deepened when they lost 4-2 at home to Parma after playing for almost the entire match with 10 men.
Lazio eased their problems with a 2-0 win at Cagliari, Livorno's 2-1 win over Bologna pulled them out of the relegation zone and Sassuolo came from behind to beat Catania 3-1 in the meeting of the bottom two.
Juventus forward Pablo Daniel Osvaldo had two first-half goals disallowed for offside with television replays suggesting that the first decision was correct and the second was not.
Buffon, equalling Dino Zoff's record of 476 Juventus appearances, rescued the visitors when he saved Emanuele Calaio's penalty in the 72nd minute after a handball by Arturo Vidal.
There was an air of inevitability about the outcome when Juventus won a free kick on the edge of the area with two minutes left and, sure enough, Pirlo curled a trademark effort over the wall and into the top corner.
Juventus, who have 75 points from 28 games and are almost certain to win their third successive title, are 17 clear of AS Roma who host Udinese on Monday (2000) when third-placed Napoli visit Torino (1800).
David Pizarro missed a penalty for Fiorentina but it did not matter as Juan Cuadrado, Alessandro Matri and Gomez, back in action after a knee injury, were on target against lowly Chievo, who replied through Alberto Paloschi.
Gomez also scored in the Europa League draw against Juventus on Thursday.
Milan slumped to a fourth consecutive defeat in all competitions and conceded four goals in a game for the second time in a week.
Milan, who had goalkeeper Christian Abbiati sent off in the fifth minute, came to life as they clawed back a 2-0 deficit, only for Parma forwards Amauri, with a brilliant back-heeled flick, and Jonathan Biabiany to win the game with late goals.
Milan, knocked out of the Champions League after an emphatic 4-1 defeat by Atletico Madrid, are stuck in 12th place with 35 points from 28 games after their 11th league defeat of a miserable campaign.
It quickly got worse when goalkeeper Abbiati clattered into Ezequiel Schelotto and suffered the so-called triple punishment of penalty, red card and automatic one-match suspension.
Midfielder Michael Essien was sacrificed to make way for substitute goalkeeper Marco Amelia, whose first job was to pick the ball out the back of the net as former Milan player Antonio Cassano fired home the penalty.
It seemed to be all over when Cassano was given too much space and added a second six minutes after halftime, but Roberto Donadoni's team then relaxed.
Adil Rami headed one back from a corner five minutes later and Balotelli levelled with a 76th-minute penalty, harshly awarded after Riccardo Montolivo went down under Joel Obi's challenge.
A heroic draw or even win was on the cards but it quickly turned to disaster when Schelotto fired over a low cross from the right and Amauri got in front of Nigel de Jong and brilliantly flicked the ball in with his heel.
Milan finished dispirited and demoralised, leaving the pitch to a howl of protests after Biabiany headed the fourth with the last move of the game.
Lazio, whose fans are furious with club president Claudio Lotito over a lack of investment in the team, won 2-0 at Cagliari with goals by Senad Lulic and Keita Balde to climb to seventh (41 points).
Teenager Balde's goal came one minute after Mauricio Pinilla missed a penalty for Cagliari, who again played in front of less than 5,000 fans due to ongoing rebuilding work at their Sant'Elia stadium.
Sassuolo's win lifted them off the bottom to 19th with 21 points, where they were replaced by opponents Catania (20). Bologna (23) dropped into the bottom three below Livorno and Chievo (24).
Sampdoria's revival was abruptly halted with a 3-0 loss at Atalanta.
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