Ronaldo has scored an incredible 31 Serie A goals this season but found himself frustrated on Wednesday night.
Luca Gagliano's close-range finish and Giovanni Simeone's powerful drive secured the win for the hosts.
Ronaldo trails Lazio striker Ciro Immobile by four goals in the battle for Europe's Golden Shoe.
Immobile scored his 35th goal of the season as Lazio beat Brescia 2-0 on Wednesday.
With one game of the Serie A season remaining, Immobile is the outright leading scorer in Europe as his latest strike moved him ahead of Bayern Munich's Robert Lewandowski, who finished the Bundesliga season with 34 goals.
Immobile is also just one goal away from equalling the Serie A record for most goals in a season.
Gonzalo Higuain scored 36 times for Napoli in the 2015-16 campaign.
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