The Italian coach is wary of the threat posed by Andre Villas-Boas' side and believes that their poor Premier League form will have little bearing on the Champions League match-up
The Serie A club host the Blues on Tuesday in the first leg of their tie after surving the 'Group of Death', progressing from Group C with Bayern Munich at the expense of Manchester City and Villarreal.
Napoli head into the home leg of the knockout fixture on the back of a convincing 3-0 victory at Fiorentina, but Mazzarri remained coy about his side's chances against Chelsea after the game.
"I do not agree that Napoli are favourites," the 50-year-old told Sky Sport Italia. "Chelsea want to do well in the Champions League and they have players who are accustomed to matches like these.
"It will be a terrible test, in order to pass it we must do something extraordinary."
The Italian coach played down Chelsea's four-game winless streak as well as reports suggesting that manager Andre Villas-Boas has a fractious relationship with his squad.
"Even if there were problems, all it takes to reassemble a group is one match," Mazzarri insisted.
"Every player wants to do well in Europe, champions and great players can make the difference at this level."