Juventus were practically crowned champions of Italy for the seventh consecutive occasion this weekend after they won 3-1 versus Bologna on Saturday evening and Napoli could only draw 2-2 on Sunday with a Torino side who had nothing left to play for.
Birthday boy Mertens took advantage of some bad defending to put Napoli ahead in the 25th minute, the Belgium worldwide stabbing the ball home after catching Nicolas Burdisso in possession just seven yards out.
Shortly after, Hamsik, who had been on the pitch for less than two minutes, fired Napoli in front again by curling into the right side of the net.
However, Juve's bitter city rivals would go on to claim a point as Lorenzo De Silvestri made the most of a lackadaisical Napoli back line to slam home a free header seven minutes from time.
Roma, meanwhile, consolidated third place and closed in on qualification for next year's Champions League by beating Cagliari 1-0 away on Sunday. Roma goalkeeper Alisson then denied the Sardinians with several outstanding saves.
Teenage star Musa Barrow gave Atalanta the lead with less than two minutes on the clock but Felipe Caicedo leveled in the 24th, thanks to great work from Luis Alberto.
Inter put last week's last-gasp 3-2 defeat to Juventus behind them, sealing victory with first-half goals from Andrea Ranocchia, Rafinha and captain Mauro Icardi. Udinese defender Seko Fofana was sent off early in the second half for a risky tackle on Ivan Perisic.
Gianluca Lapadula appeared to have turned the match around four minutes later but Goran Pandev was sent off shortly after for a two-footed tackle on Bruno Gaspar.
Verona and Benevento have already been relegated.