It looked as though Juve were going to preserve their four-point advantage at the summit with four games to play and edge closer to a seventh successive Scudetto only for Koulibaly to throw the race wide open.
Compounding Juve's last-gasp defeat is the fact Massimiliano Allegri's men arguably have the tougher run-in of the two sides, with the Derby d'Italia away to Inter coming up next Saturday, and a clash with Champions League semi-finalists Roma awaiting in the penultimate round of fixtures.