Juventus lost their first Serie A game of the season as Stefano Sturaro and Goran Pandev sealed a memorable 2-0 win for Genoa at Stadio Luigi Ferraris on Sunday.
Genoa activated their obligation to buy Sturaro from Juventus for €16.5million in February, less than two weeks after he joined on loan and he marked his first appearance of his second spell with the opener after 72 minutes.
That was followed nine minutes from time by Pandev's fine strike past Mattia Perin to secure a deserved victory for the hosts.
Cristiano Ronaldo was rested after his midweek heroics against Atletico Madrid and Juve sorely missed his attacking threat in what was an insipid display.
Genoa came close to taking a surprise lead after 15 minutes but Perin showed superb reactions to paw away Antonio Sanabria's powerful drive from 12 yards.
The hosts then had a penalty award overturned by VAR on the half-hour mark after Christian Kouame was deemed to have handled the ball before it struck Joao Cancelo's wrist.
Juve thought they had taken the lead with their first shot on target after 55 minutes through Paulo Dybala. Referee Marco Di Bello was instructed by VAR, however, that there had been an offside in the build-up and the goal was ruled out.
The hosts took full advantage of that reprieve in a dramatic final 20 minutes.
Just moments after replacing Darko Lazovic, Sturaro sent a curling effort into Perin's bottom corner from 20 yards, which the Juve goalkeeper will feel he should have done better with.
Things got even better for the hosts in the 81st minute when Pandev collected Kouame's pass and fizzed a low strike past Perin.
Allegri's side scarcely looked like clawing their way back into the game in the closing stages and could end the weekend with their lead at the summit cut to 15 points if Napoli overcome Udinese later on Sunday.