Antonio Conte's champions-elect failed to find a killer second goal against 10 men and dropped two vital points after huge mistake from Buffon.
Claudio Marchisio had given Antonio Conte's side an early advantage, but Buffon took a poor touch to allow Bertolacci to slide into an empty net. Coupled with AC Milan's win over Atalanta, the gap at the top is now back to just a single point.
Bianconeri boss Antonio Conte will be furious at his troops, who failed to take advantage of their obvious superiority and have handed Milan some momentum in the tittal race.
An end-to-end opening suited the home side whcih got off to a great start on eight minutes when Marchisio got on the end of a drifting Andrea Pirlo cross to nod past the flat-footed Massimiliano Benassi.
The visiting side nearly found itself two down on 26 minutes. A lovely passage of play saw the influential Marchisio dance through the middle and flight a pass to Arturo Vidal, but the Chilean fired wide of the upright from 12 yards out.
Marchisio then went close himself, Benassi turning the Italy international's bouncing 25-yard effort round the post.
The visiting Giallorossi started to knock a few passes around before the break, but entered the dressing room without mustering a shot on target.
Juventus emerged for the second period seemingly content to protect the one-goal cushion and the task was made easier on 54 minutes when Lecce striker Juan Cuadrado earned a second yellow card for a rash tackle on Uruguayan Martin Caceres.
Vidal almost exploited the extra man immediately, the Juve midfielder striding onto a Vucinic header only to hit a tame effort at Benassi.
Nearing the 70 minute mark, a comfortable Bianconeri nearly doubled their lead, Fabio Quagliarella hitting a delightful left foot volley just over from the edge of the area before testing a sprawling Benassi moments later with a low curling strike.
Juventus was struggling to put Lecce to the sword, however, and nearly paid the price when Luis Muriel almost latched onto a partially cleared free kick only to be denied by Buffon.
Muriel's half chance stung the home side into action and substitute Alessandro Matri crowned a fine run down the left flank with a firm angled shot - only to see Benassi palm the effort to safety.
Then Buffon's calamitous moment came with five minutes left on the clock. The keeper seemingly had acres of space, but took a shocking touch to allow Bertolacci to nip in and gleefully pass home into the empty net.
Marchisio went close for Juve in a desperate finale, but the visiting team hung on to gain an unlikely draw, which will go some way to soothing relegation fears.
Juventus, meanwhile, will enter the final games of the season just one point ahead of AC Milan.
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