Inter Milan's lead in the Italian league was trimmed to seven points after Jose Mourinho's side lost 1-0 at Napoli and city rivals AC Milan beat Palermo 3-0.
Yesterday, Napoli's Marcelo Zalayeta scored the only goal of a high-tempo, but it was an uninspiring match in which both sides failed to create many chances.
Earlier Kaka converted two penalties to lead Milan to victory over Palermo, while Juventus was held to a 2-2 draw by last-place Reggina.
Reigning champion Inter remains top of Serie A with 74 points, while AC Milan is second with 67, Juventus has 65 and Fiorentina 58.
"It was a victory we had to work for and were not expected to get," Zalayeta said. "We have failed to win matches that were easier than that this season," he added.
Inter lacked imagination and was outmuscled by a more determined Napoli side, who won its first match since January and the first since ex-Italy coach Roberto Donadoni took over in February.
"Obviously it is emotional to win, especially against a team like Inter, who is top of the table," Donadoni said. "I think our defense did great, especially with rebounds and the second ball into the box.