Napoli survived a second-half fightback from Brescia to move into fourth place in Serie A with a 2-1 home win on Sunday as Mario Balotelli opened his account for the visitors.
The result leaves Napoli in fourth place on 12 points, six behind leaders Inter Milan, while Brescia are down in 14th on six points.
Mertens' powerful finish put the hosts ahead early on before Manolas, who had an earlier goal disallowed for handball, doubled their advantage with a glancing header in first-half stoppage time.
Brescia's 19-year-old midfielder Sandro Tonali looked to have pulled one back in style with a long-range screamer before it was chalked off for a foul in the build-up following a Video Assistant Referee review.
Balotelli scored his first goal for his hometown club when he rose to head in a Tonali corner with 23 minutes remaining, but Brescia could not find a leveller despite a period of sustained pressure in the closing stages.