Napoli restored their one-point advantage over champions Juventus who had beaten nine-man Chievo 2-0 on Saturday with goals by Sami Khedira and Gonzalo Higuain.
The game in Naples proved to be a battle with Rodrigo Palacio nodding Bologna ahead after 25 seconds before an Ibrahima Mbaye own goal four minutes later pulled the hosts back at their Stadio San Paolo.
Mertens edged Napoli ahead on 37 minutes with a penalty awarded for a foul on Jose Callejon. The Belgian then claimed his 13th goal of the season with a spectacular curling effort on 59 minutes.
Maurizio Sarri's side have 57 points from 22 games, one more than Juventus.
"We cannot compare ourselves to Juventus," said Sarri. "They are clearly superior in every way. Juve win 90 per cent of their matches."
"Our aim is to play beautifully. We like playing good football, enjoying ourselves and entertaining."
Inter Milan missed a chance to move third with a frustrating 1-1 draw at promoted SPAL.
Inter remain fourth, two points behind Lazio who stay third despite losing to AC Milan in the San Siro. Roma are just three points behind Inter in fifth before playing at Sampdoria. Milan moved seventh after winning a third consecutive game for the first time this season.
"It's incredible. A free kick deflected in by an arm," complained Lazio coach Simone Inzaghi after his side's first loss in nearly two months.
Milan and Lazio meet again in Italian Cup semi-final action midweek.
- 'Bitter taste' -
==================
Earlier Alberto Paloschi headed in a last-gasp equaliser to grab a precious point for SPAL after Inter had taken the lead on 48 minutes when defender Francesco Vicari turned the ball into his own net.
Despite trailing for most of the second half, SPAL proved dogged and their pressing paid off when former Milan forward Paloschi turned in a Mirco Antenucci cross on the stroke of full-time.
Spalletti introduced on-loan Barcelona midfielder Rafinha for the final minutes of the game but the Brazilian failed to lift his team against a SPAL side still in the relegation zone.
Crotone and Cagliari drew 1-1 in a relegation battle overshadowed by a video assistant referee (VAR) controversy.
Crotone were awarded a soft penalty and Fabio Pisacane received a straight red card for a high tackle, with a late Federico Ceccherini goal ruled offside using VAR.
"Perhaps the VAR monitor was bent... Perhaps the referee was confused as to who was defending and who was attacking. I have to laugh, because if anyone was offside it was the team with the white jerseys, except they were the ones defending!" said Crotone coach Walter Zenga.
Disclaimer: No Business Standard Journalist was involved in creation of this content
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