They last won the league in 1990 with a Diego Maradona-inspired side adding to their first title three years earlier.
But Boulaye Dia equalised with a left-footed drive six minutes from time.
It silenced an expectant crowd who made a cacophony of noise and set off blue smoke flares inside and outside the stadium, where thousands of fans lined the streets, when Napoli went ahead.
Despite the disappointment Spalletti’s side are almost certain to win the title, with just two points needed from their final six games to secure a third Serie A crown – and first without talisman Maradona.
Their next chance to secure the title will come when they travel to Udinese on Thursday (19:45 pm), but they could also win it before then if results go their way.