Two first-half goals for Monza proved enough to give them their second win of the season against Juventus to add to the Bianconeri's problems.
Patrick Ciurria and Dany Mota struck for Raffaele Palladino's men as they secured a 2-0 win at Allianz Stadium on Sunday to follow up their 1-0 win earlier in the season in Palladino's first game in charge.
Juve only beat Monza 2-1 in the Coppa Italia 10 days prior but struggled to deal with their energetic visitors here, as they had done in their defeat at Stadio Brianteo in the reverse league fixture in September.
It is a further blow to Juve's hopes of Champions League qualification following their 15-point penalty related to the club's transfer dealings imposed earlier this month.
Monza were denied the lead when Gianluca Caprari's strike was ruled out for offside after a VAR check, but they were not made to wait long to rectify that as Ciurria raced onto a pass from Jose Machin before firing into the roof of the net in the 18th minute.
It was two for the visitors six minutes before half-time as Carlos Augusto went on a mazy run through the Juve defence before feeding Mota, who rounded Wojciech Szczesny and finished calmly.
Massimiliano Allegri opted to bring Arkadiusz Milik on for Moise Kean early in the second half as the hosts looked to find a way back into the game, and Michele Di Gregorio did well to keep out a fierce effort from the Pole.
The 25-year-old stopper produced his finest save of the game in the 74th minute when he tipped an Angel Di Maria effort from long range wide when it had been arrowing towards to top corner of his goal.
Gleison Bremer had a goal disallowed with just over 10 minutes remaining as Juve toiled and ultimately failed to get back into the contest.