Mertens – who also scored in Napoli's 2-0 win over Liverpool last month – twice put his side in front on Wednesday, only to see teenage prodigy Erling Haaland level for the hosts on both occasions.
The terrific Mertens then turned provider, setting up substitute Lorenzo Insigne for the 73rd-minute winner, a strike that came swiftly after Haaland got his second.
Napoli retook the lead so quickly, they were still showing a replay of Salzburg’s goal 😅 pic.twitter.com/6jHKHKsGqI— Bleacher Report Live (@brlive) October 23, 2019
Haaland thought he had put the hosts in front in the eighth minute with an instinctive left-foot finish, but the celebrations of the home fans were cut short by a VAR-assisted offside decision.
At the other end, Mertens was causing problems and he fired Napoli in front after racing inside the penalty area and thrashing a fierce strike from a tight angle past Salzburg goalkeeper Cican Stankovic.
Salzburg responded well to falling behind, though, the outstanding Haaland forcing two good saves from Napoli's Alex Meret before he also shot wide.
Meret also kept out a terrific Patson Daka effort before Haaland got the goal his first-half performance deserved when Hwang Hee-chan skipped past Kevin Malcuit and was then brought down inside the penalty area by the defender.
The striker coolly slotted away the spot-kick as both sides went to the break level and after a tame beginning to the second half, the match exploded into life when Mertens, left free at the back post, turned a 64th-minute cross in - a goal that saw the Belgian take his tally to 116 goals with Napoli, to surpass club legend Diego Maradona's 115.
Haaland levelled again, scoring a thumping header, but Napoli were not to be denied, Insigne's clever finish settling matters in Austria.