Napoli fell four points behind second-place Inter Milan in Serie A on Sunday after slumping to a 2-1 loss at Lecce in which leading scorer Edinson Cavani was sent off.
Daniele Corvia gave Lecce the lead with a penalty before he was sent off himself. Giuseppe Mascara equalized, but any hopes Napoli had of winning disappeared down the tunnel with Cavani 16 minutes from time.
His fellow Uruguayan Ernesto Chevanton then hit a spectacular, but controversial, winner with a half volley from the edge of the area that Napoli claimed had not crossed the line.
''Lecce won with a great shot, but there was a penalty before that we should have been given,'' Napoli coach Walter Mazzarri said.
''It has been a long season for Cavani since the World Cup and he's tired. He got frustrated today because the more he tried to score, the less likely it seemed.''
Inter meanwhile bounced back from losing its league title to city rival AC Milan on Saturday, with a convincing 3-1 win at home to Fiorentina.
To rub salt into Cavani's wounds, his rival for the leading scorer title - Antonio Di Natale - scored twice in Udinese's 2-0 win over Lazio to take his tally to 28, two more than Cavani.
Lazio's loss means Napoli is guaranteed a place in next year's Champions League, but if it loses its last two games and Udinese wins both, then Udinese will take the automatic qualification place as it won both matches between the sides this season.
At the bottom of the table Brescia was relegated after a 2-1 loss to Catania. Also Bologna and Parma drew 0-0, while Cesena won 2-0 away to Cagliari.
Lecce had the best chance of a scrappy first half early on, but Gianni Munari shot into the side netting from close range.
Soon after the break the host took the lead, when Corvia fired home a penalty after Ruben Olivera was brought down by Hugo Campagnaro.
Soon after the striker was sent off when he was booked for a second time for tripping Ezequiel Lavezzi. It didn't take Napoli long to take advantage of its numerical superiority when Mascara was left unmarked to head in Juan Zuniga's cross from close range.
In the 74th Cavani allowed his frustrations at not scoring boil over and received a second yellow card for taking his protests to the referee too far.
Lecce took advantage two minutes from time when Chevanton struck. The ball bounced over the line, Napoli goalkeeper Morgan De Sanctis quickly grabbed the ball and claimed it hadn't, despite replays showing the opposite.
''The ball looked like it had gone over the line,'' Lecce coach Luigi De Canio said. ''The referee took the chance to confirm it and his assistant let him know over the radio.''
At the San Siro, Fiorentina had the first chance of the match, but Julio Cesar tipped Alberto Gilardino's volley onto the bar.
Two minutes later Inter hit the woodwork after Pazzini turned and shot, but Artur Boruc flicked his effort onto the bar.
There was nothing the Polish goalkeeper could do to stop Pazzini from opening the scoring in the 25th. Samuel Eto'o crossed and Pazzini held of Cesare Natali to turn and shoot low past Boruc.
Inter was two goals ahead three minutes later when Cambiasso flicked Eto'o's freekick over Boruc for his seventh goal of the season.
''It was a difficult match to get to grips with,'' Inter coach Leonardo said, ''but the way we took control and adapted is a step forward for our maturity and cohesion as a team.''
Just before the break Alessio Cerci stabbed Gaetano D'Agostino's low cross wide and Boruc used his legs to stop Maicon's low drive.
Gilardino gave Fiorentina some hope when he beat the offside trap to volley past Cesar, but moments later Coutinho curled a freekick over the wall to take Inter's lead back to two goals.
''I'm pleased for him,'' Leonardo said. ''He's a good player with excellent technique, but he hasn't been able to find a place in the team because it has changed so much.''
Udinese gave Lazio notice of its attacking intent in the 29th, when Alexis Sanchez and Mauricio Isla linked up to create a chance for Di Natale that he hit wide.
Six minutes later though Sanchez and Di Natale took advantage of more lax Lazio defending to open the scoring. Sanchez chipped over the defense to Di Natale, he took one touch and shot past Fernando Muslera.
Udinese doubled the lead before halftime and again Di Natale capitalized on build up play by Isla and Sanchez. Sanchez again sprung Lazio's offside trap, Isla squared to Di Natale and he only had to stroke the ball into the empty net.
Lazio brought on captain Tommaso Rocchi and he made an immediate difference. Udinese defender Gabirele Angella clumsily brought him down and was sent off. Mauro Zarate tried to chip Samir Handanovic with the penalty, but the Slovenia 'keeper stood tall to make his sixth penalty save of the season.
Zarate made amends when he crossed for Libor Kozak to volley Lazio's first and while the Rome side came close again it couldn't find another way past Handanovic.
''What really pleases me is the fact that this team is very young,'' Francesco Guidolin said. ''Di Natale uses his experience to help the team, but we've not achieved anything yet and know that the last two games will be very tricky.''