Napoli will face Juventus in Wednesday's final at the Italian capital's Stadio Olimpico.
Juventus are two points behind Inter Milan after three games and could be overtaken by city rivals Torino who host promoted Lecce on Monday.
Dries Mertens headed in the equaliser after 85 minutes in Naples to surpass former Argentine ace Diego Maradona, who hit 81 league goals between 1984 and 1991.
Dries Mertens moved level with Diego Maradona in Napoli's all-time Serie A scoring list after bagging his 81st league strike in their 2-0 win at Frosinone.
Romelu Lukaku and Dries Mertens as Belgium ran rampant with further goals from Thomas Meunier, Eden Hazard, Yannick Carrasco and an own goal against Estonia to make it four wins from four for the Belgians in 2018 World Cup qualifiers