Empoli stunned Napoli with a 2-1 home win on Wednesday to boost their survival hopes and leave the visitors 18 points adrift of Serie A leaders Juventus.
Giovanni Di Lorenzo's second-half winner lifted the Tuscan side out of the relegation zone at Bologna's expense ahead of the remaining Week 30 fixtures.
Napoli remain second in the standings on 63 points with a 10-point advantage over third-placed Inter Milan, but Juventus, who beat Cagliari 2-0 on Tuesday, have a huge lead over their nearest rival.
Empoli, who are unbeaten in their last five home matches, have risen to 17th place on 28 points, one ahead of Bologna, who face Atalanta in Bergamo on Thursday.
"We were off the pace from the first minute to the last, it was a deserved defeat," Napoli coach Carlo Ancelotti admitted to Italian broadcasters.
Ancelotti made five changes to the team that won 4-1 away at AS Roma last weekend, but his side were behind within 28 minutes after Farias' shot deflected off the arm of Zielinski to wrong-foot goalkeeper Alex Meret.
Meret then came to Napoli's rescue with a point-blank save to deny Francesco Caputo, before Zielinski levelled just before the break with a powerful long-range strike that he refused to celebrate against his former club.
Empoli came out strongly at the start of the second half and were back in front within seven minutes when Di Lorenzo rose to nod in a corner from the impressive Ismael Bennacer.
Farias had a chance to put the result beyond doubt for the hosts when he raced through on goal with a quarter of an hour remaining, but he was denied by another excellent save from Meret.
Napoli next face Genoa at home in Serie A on Sunday, before they travel to face Arsenal in the first leg of their Europa League quarter-final clash at the Emirates next Thursday.
"We must roll up our sleeves and think about the next game," Ancelotti added.
"Defeats are never physiological, we were disorganised, distracted, we didn't follow our plans. It's not a problem with the defence, it was a below-par performance in general."