Later Mario Mandzukic sealed the three points on the hour mark in a simple warm up for Tuesday’s crucial Champions League visit of Valencia.
The pair also scored both in Juventus’ 2-0 win over AC Milan before the international break, and Ronaldo’s strike was his seventh in eight games in all competitions for his new club, for a total of 10 goals this season.
“Cristiano does things that catch opponents off guard and that’s exactly what we need,” said coach Massimiliano Allegri.
“Sometimes we play one pass too many and the moment is lost.
“Cristiano moves and thinks so quickly, he saw Douglas Costa coming out the corner of his eye and pulled the pass back. We need to all improve in that sense, being able to think ahead to the next move.”
SPAL, fighting relegation, were no match for Allegri’s reigning champions who are nine points ahead of Inter Milan and Napoli, with Carlo Ancelotti’s side gunning to close the gap when they play bottom club Chievo on Sunday.
Bustling Ronaldo, 33, also played a role in Juve’s second scored by Mandzukic, who was wearing the captain’s armband for the first time.
Ronaldo latched onto a long ball and pulled it back for Douglas Costa, whose shot was cleared into the path of the Croatian, who gobbled up he opportunity to win the match.
The 33-year-old Ronaldo before moving to Juventus won the Champions League four times with Real Madrid.