The reigning champions were off-colour at the Stadio Luigi Ferraris and an understrength team struggled to dominate an enterprising Genoa side, but Pirlo's last-gasp effort saw Juve move further clear of second-placed Roma, who are in action on Monday.
Also on Sunday night, Fiorentina regained fourth spot thanks to a 3-1 win over Chievo which included Mario Gomez's first goal at the Stadio Artemio Franchi.
Juan Cuadrado gave the Viola the lead in the 11th minute before Alessandro Matri doubled their advantage before the break.
Alberto Paloschi took his third big chance of the match to reduce the arrears, but it was not enough as, after David Pizarro missed a penalty for the hosts, Gomez notched his first goal on home soil to seal the win.
Earlier on Sunday, AC Milan's miserable recent run continued as old boy Antonio Cassano inspired Parma to a 4-2 victory at San Siro.
The pressure has been building on coach Clarence Seedorf following back-to-back Serie A defeats to Juventus and Udinese while the seven-time European champions limped out of the Champions League this week after losing 5-1 on aggregate to Atletico Madrid.
And there was no respite for the stuttering Rossoneri on Sunday as they crashed to another defeat.
Milan were in trouble early on when goalkeeper Christian Abbiati was sent off in the fifth minute for conceding a penalty, which Cassano slotted home to open the scoring.
Cassano then doubled Parma's lead in the 51st minute before Milan hit back to make it 2-2 with goals from Adil Rami and Mario Balotelli, who levelled from the spot.
However, Seedorf's men could not build on that fightback and Amauri and Jonathan Biabiany scored late on to ensure all three points went back to Parma as the Ducale club reached fifth place on the back of a third straight win.
Milan, meanwhile, drop to 11th after Atalanta climbed into the top half of the table with a 3-0 win over Sampdoria, goals from Carlos Carmona, Giacomo Bonaventura and German Denis earning a third successive win for the Nerazzurri.
Lazio, who lost at home to Atalanta last time out, climbed to seventh after securing a comfortable 2-0 victory over 10-man Cagliari.
Senad Lulic and Keita Balde Diao were the goalscorers, with Cagliari not helping their cause by missing a second-half penalty through Mauricio Pinilla when only one goal down before Daniele Conti was sent off with 13 minutes to go.
In the fight to avoid relegation, Sassuolo came from behind to beat fellow strugglers Catania 3-1.
Sassuolo were bottom of the table and without a win in eight attempts heading into the basement bout at the Stadio Citta del Tricolore but victory over Catania saw them leapfrog the Sicilians and take 19th place.
Gonzalo Bergessio gave the visitors the lead before half-time but Simone Zaza and Simone Missiroli turned the game on its head before Nicola Sansone put the result beyond doubt at the death.
Sassuolo are now just three points behind 17th-placed Livorno - who left the relegation zone with a 2-1 victory over Bologna - while Catania prop up the standings with only 20 points.
Livorno picked up three points despite finishing with nine men.
Two goals in quick succession at the start of the second half from Marco Benassi and Paulinho put Livorno in charge, but they were left holding on in the end after losing Ibrahima Mbaye and Innocent Emeghara to red cards as well as seeing Bologna pull a goal back through Lazaros Christodoulopoulos' penalty.