Juve have won the last six league titles under Antonio Conte and Massimiliano Allegri, taking the crown with a four-point advantage over Roma in 2016-17.
They sit one point off the pace after 12 games this season, though, with Maurizio Sarri's unbeaten Napoli leading the way after 10 wins and two draws.
Inter are also in close attendance after an undefeated start of their own, and Marotta believes it is Luciano Spalletti's side that will push Juve the hardest for the Scudetto.
"I think that, given they have no international and European commitments, Inter are the ones who can tailor their preparation the best," Marotta told Domenica Sportiva.
"They're a very solid team, they're not spectacular but they're clinical and getting good results. So I think Inter are the ones who are the most fearsome."
He added: "At the moment, we're a point behind Napoli, but we have a point more than last year. I have to say that Napoli's journey has been extraordinary so far.
"[Napoli's] draw [against Chievo on Sunday] calmed the league down a little, but they're on an extraordinary journey while ours has been ordinary so far.
"At the same time, we're doing perfectly well when compared to last year, so we know that getting up there and staying there is very difficult.
"We apply our model, a particular model where suffering is important."