Dortmund boss believes Schalke or Gladbach could win title
Jurgen Klopp's side were superb on the domestic stage last season, pipping Bayern Munich to the Bundesliga title, before thrashing the Bavarians 5-2 in the the DFB-Pokal final.
However, despite his side's dominance in the last campaign, Zorc feels that there are several teams capable of preventing his club from winning the league for a third consecutive year.
"It would be naive to think that you can dominate the Bundesliga for years," he told Der Westen.
"There are a few other clubs that have good teams and could challenge. Schalke and Borussia Monchengladbach could both be surprise contenders.
"If we focus on facts and reality, it is no understatement to say that Bayern Munich are the favourites to win the Bundesliga."
Having pinched Borussia Monchengladbach forward Marco Reus from under the noses of their Bavarian rivals, Zorc also discussed how the positive image of his club is helping entice young players.
"We are interested in young players who are hungry for success and will continue to evolve. This is a prime destination because we are proven at developing young players well."
Borussia Dortmund begin their Bundesliga defence with a home match against Werder Bremen on August 24.