Miralem Pjanic’s strike put the hosts in front on the stroke of half-time before Ronaldo headed his third goal of the campaign with 12 minutes remaining.
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Juve lead the table on 16 points but Inter Milan can return to the summit when they travel to Sampdoria later on Saturday.
SPAL goalkeeper Etrit Berisha made a string of impressive saves to keep the score down as the visitors fell to their fifth defeat in six matches.
They stay level on three points with Sampdoria at the foot of the table.
(Reuters/NAN)
