Juve's Champions League hopes are no longer in their hands as they head for 11th-placed Bologna needing a win and hoping rivals Napoli and AC Milan slip up.
Lorenzo Insigne and Victor Osimhen struck for the hosts before Simy pulled one back, but Dries Mertens' free kick restored Napoli's two-goal cushion before the break.
Nigeria international Simy's double continued a superb run of form that makes him the top scorer in Europe's top-five leagues since the start of March with eight goals.
Zlatan Ibrahimovic's AC Milan need to improve their home form to keep in the running for a first 'Scudetto' since 2011, having not won any of their last five games in all competitions at the San Siro.
Champions League rivals Atalanta and Napoli go head-to-head on Sunday before the Bergamo side host Real Madrid in this year's elite European tournament.
Inter committed more players forward as the match wore on, but the pressure was soaked up by the home defence as the visitors finished the match without scoring for the seventh successive trip to Juventus in all competitions.