Third-placed Lazio lost 2-1 at AC Milan amid controversy as Patrick Cutrone appeared to use his arm to score the hosts' opener at San Siro and the goal stood despite the presence of the video replay system (VAR).
The VAR denied Crotone a last-minute winner at home to 10-man Cagliari, the game ending 1-1, and a bizarre miss by Genoa forward Luca Rigoni cost his team a point at home to Udinese as they lost 1-0.
Napoli, briefly dislodged by Juventus who played on Saturday and won 2-0 at Chievo, now lead with 57 points from 22 games, one ahead of the Turin side.
Juventus opened up a 10-point gap over third-placed Lazio while Inter are a further two points behind in fourth.
Napoli were stunned when Rodrigo Palacio took advantage of dreadful marking to give Bologna the lead after 25 seconds but they were gifted an equaliser by Ibrahima Mbaye who turned an attempted clearance into his own goal.
The hosts were then awarded a soft penalty which Mertens converted in the 37th minute but there was no question about the Belgian's second goal in the 59th when he cut inside a defender and curled an exquisite effort into the net.
"We aim for beauty, to enjoy ourselves and give enjoyment," said Napoli coach Maurizio Sarri, who was a bank worker before becoming coach. "If that produces results, so much the better. If I only wanted to work, I would have stayed at the bank."
Inter, leaders in early December, slipped further behind after a game in which both goals were scored by their opponents.
SPAL's Francesco Vicari turned Joao Cancelo's cross into his own net to gift Inter the lead and the visitors were on the verge of their first win in all competitions since Dec. 3 until Alberto Paloschi's diving header levelled the scores.
AC Milan went ahead after 15 minutes when Cutrone turned in Hakan Calhanoglu's free kick but television replays suggested the ball came off his arm. However, the referee did not ask to review the incident.
Adam Marusic levelled five minutes later but Giacomo Bonaventura headed in one minute before halftime to give Milan their third successive league win.
Lazio coach Simone Inzaghi approached Cutrone at the end and asked if he had used his hand, but the player pointed to his shoulder.
The VAR system was used three times at Crotone, firstly to award the hosts a penalty which was converted by Marcello Trotta in the 29th minute and then to confirm a red card for Cagliari's Fabio Pisacane for a late tackle.
Luca Cigarini levelled for the Sardinians with the last kick of the first half and they held out until the final minute when Federico Ceccherini headed in for Crotone but the referee changed his mind and disallowed the goal after consulting the pitchside monitor.
Valon Behrami's second-half goal gave Udinese a 1-0 win at Genoa although they had to cling on after Samir was sent off shortly afterwards -- the referee changing the card from yellow to red after checking with the VAR.
Genoa should have equalised in the 88th minute when Adel Taarabt's cross found Rigoni who only had to tap the ball in at the far post but instead contrived to clear it off the line.
Alessandro Florenzi had a first-half penalty saved by Emiliano Viviano as AS Roma lost 1-0 at home to Sampdoria to extend their winless league run to six games. Duvan Zapata scored the winner in the 80th minute.
Fiorentina's season took a turn for the worse with a 4-1 home defeat by next-to-bottom Verona and Torino thumped bottom club Benevento 3-0.
(Writing by Brian Homewood in Berne; Editing by Clare Fallon)
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