A first-half winner from Jose Callejon handed Napoli the points from a 1-0 win at Inter Milan on Sunday that closed the gap on Roma in second to a point.
Maurizio Sarri's men travelled to the San Siro buoyed by Roma's earlier 3-1 derby defeat to Lazio that left the capital contenders nine points adrift of leaders Juventus.
By the end of a San Siro sizzler that saw Stefano Pioli's outplayed hosts stage a brief, second-half comeback, Callejon's 43rd minute opener proved the difference to a win that put Napoli in firm contention for the second automatic Champions League qualifying place.
Roma remain second at nine points behind Juventus with four games to play but Luciano Spalletti's men are now just one point ahead of Napoli, who remain seven points ahead of Lazio, in fourth.
Yet coach Maurizio Sarri said his side's fate lies with their opponents, telling Premium Sport: "I've never made predictions and I'm not about to start now. The situation is determined by our rivals, who, if they all win, will give us no chance of fighting our way back."
Inter's fourth defeat in a six-match winless run, meanwhile, left the Nerazzurri in seventh, but now three points behind city rivals AC Milan and eight behind fifth-placed Atalanta.
But Pioli hasn't given up hope of a Europa League place, hoping they can finish sixth to claim the Italian Cup winner's allocation that, in all likelihood, will be vacated by finalists Juventus and Lazio.
"At the moment, the results are negative but we can still finish sixth in the table. That's our objective," said Pioli.
"It wasn't all negative tonight. I think we played well given the calibre of the side we played. They controlled the ball better than us, and our passing was off at times."
- Disappointed and angry -
Earlier, Milan spurned the chance to steal a march, suffering a second league setback since their takeover by a Chinese consortium.
Milan travelled to relegation-haunted Crotone looking to make up for a shock 2-1 San Siro defeat to struggling Empoli last week.
But for the second game in succession since the Rossoneri Sport Investment Lux bought the seven-time European champions from long-time owner Silvio Berlusconi, Vincenzo Montella's side disappointed in a 1-1 draw.
"We should be disappointed and angry for the result," Montella told Mediaset Premium.
In the June 2 Cup final, Juventus, set for a Champions League spot, face Lazio -- who look set to finish fourth and book a spot in Europe's second tier club competition.
"We still believe we can reach the Europa League," added Montella. "We've still to face Atalanta and sixth place still offers a way in."
In the fight to secure the second automatic Champions League spot, Roma blundered.
Keita Balde struck twice, either side of a Dusan Basta strike and Daniele De Rossi's goal for Roma, as a dominant Lazio ruined Francesco Totti's 'final' derby to give Napoli a pre-game boost.
Elsewhere, former Roma striker Mattia Destro struck in either half to complete a brace as Bologna secured their top flight survival with a comprehensive 4-0 win over Udinese.
It gave Roberto Donadoni's men a 13-point lead over third-from-bottom Crotone and Bologna forward Simone Verdi said: "We've deserved to stay up this season, so it's only right we celebrate."
Genoa and Empoli, however, face a nervous end to the season after defeats to Chievo and Sassuolo respectively left them just above the drop zone.
While rock-bottom Pescara's relegation last week was hammered home on Sunday by a 1-0 defeat away to Cagliari, fellow strugglers Crotone and Palermo can dream of top-flight survival for another week.
A point from their draw with Milan left Crotone third from bottom but now just four points behind Empoli.
But Crotone coach Davide Nicola remains unconvinced, saying: "We're neither closer, nor further away (from safety."
Palermo saw goals from Alessandro Diamanti and Haitam Aleesami, at the death, secure a 2-0 home win over 10-man Fiorentina -- only their fourth of the season. It left the Sicilians second from bottom and 10 points behind Empoli.