Juve's third defeat of a season in which they are hoping to secure a record sixth consecutive 'scudetto'.
A quickfire brace from Giovanni Simeone, the son of Atletico Madrid coach Diego, opened up the Serie A title race on Sunday as Genoa claimed a stunning 3-1 win over leaders Juventus.
Juve's third defeat of a season in which they are hoping to secure a record sixth consecutive 'scudetto' left the Turin giants just four points ahead of on-form AC Milan, with several other challengers close behind.
Coach Massimiliano Allegri admitted the defeat was a blow in a game that saw Leonardo Bonucci and Dani Alves added to the club's lengthy injury list.
"It's a blow and has brought us back to earth with a bump," said Allegri. "I'm angry, but no team is capable of winning all the time."
Juventus travelled to the Luigi Ferraris Stadium without a number of key players, with forwards Paulo Dybala and Marko Pjaca and defender Andrea Barzagli notably all in the treatment room.
Experienced defender Giorgio Chiellini was on the bench recovering from a knock, meaning a start in the back three for Swiss wing-back Stephan Lichtsteiner.
With Argentine striker Gonzalo Higuain in the same straits due to a muscle problem, Allegri was forced to play Juan Cuadrado up front alongside Mario Mandzukic.
Genoa sensed an upset, and were 2-0 up by the 13th minute thanks to a fine Simeone double.
"We're delighted to have beaten the strongest team in Italy, it gives you a special kind of satisfaction," said Genoa coach Ivan Juric.
"We had a great game and everyone was completely focused. I want us to kick on from this and have more confidence in ourselves."
For the opener, Lichtsteiner was outfoxed by Diego Laxalt in midfield and the fleet-footed Uruguayan motored away to set up Luca Rigoni for a drive that Gianluigi Buffon did well to parry.
But Lucas Ocampos was quick to the rebound and when Buffon stopped the ball again, Simeone pounced to beat the keeper at the second time of asking.
When Simeone was left unmarked on the edge of the box by Moroccan defender Medhi Benatia, it left the 21-year-old Argentine free to get down low and direct a diving header past Buffon, although Darko Lazovic deserved plenty of credit for turning Alex Sandro inside out prior to the cross.
Genoa were looking dangerous every time they moved upfield, and a nervous Juve conceded a third before the half-hour.
A chaotic corner was knocked down for Rigoni to fire a volley goalwards and when Alex Sandro attempted a clearance, the Brazilian only managed to shin the ball over the line and into the roof of his own net.
Juve avoided a total whitewash when Miralem Pjanic stepped up to curl a superb free-kick past Mattia Perin eight minutes from the end.
Elsewhere Crotone moved off bottom spot thanks to a 1-1 home draw with Sampdoria, thanks also to Palermo's earlier 1-0 defeat to Lazio that sent the Sicilians to rock bottom.
Lazio, who host Roma in the city derby next week, are up to third, just five points behind Juve.
Atalanta's fine form continued with a 2-0 win at Bologna thanks to goals from Andrea Masiello and Jasmin Kurtic either side of the interval that moved the Bergamo-based side up to fourth, also just five points off the lead.
Cagliari had Marco Sau's 58th-minute goal to thank for a 2-1 win at home to Udinese after Seko Fofana's superb 51st-minute strike levelled for the visitors following Diego Farias's 35th-minute opener from the spot.
Roma, currently fifth, can move to within four points of Juve with a win at home to Pescara later on Sunday.
On Monday, Napoli will look to close their nine-point gap to Juve when they host Sassuolo, while Inter Milan host Fiorentina.