Napoli will have watched on with glee as Juventus were held to a scoreless draw at struggling SPAL.
Juventus failed to win for the first time in 13 Serie A matches as lowly SPAL earned a 0-0 draw at Stadio Paolo Mazza on Saturday.
Heavily favoured to strengthen their grip on the Scudetto against the relegation-threatened Biancazzurri, Juve could instead have their lead trimmed back to two points on Sunday when Napoli host Genoa.
Alex Sandro's early blocked shot and a marginally misdirected Paulo Dybala free-kick were the best of few opportunities extracted from the plucky hosts.