)
Belgium forward Mertens slotted home his 122nd goal for Napoli four minutes before half-time to level the scores at 1-1 at the Stadio San Paolo and put them 2-1 ahead on aggregate.
Christian Eriksen had opened the scoring for Inter straight from a corner in the second minute.
Mertens, 33, overtakes Marek Hamsik and moves seven clear of Diego Maradona, who scored 115 goals in all competitions over his seven tumultuous years in southern Italy.
Mertens moved to Napoli from PSV Eindhoven in 2013 and has become so settled in Naples that fans have nicknamed him 'Ciro', a hugely common Neapolitan name.
)
)
)
Should Napoli come through the semi-final they will face Juventus in Wednesday's final in Rome following the Italian champions away goals victory over AC Milan on Friday night.
td/dj