Duarte heaped praise on Pirates after his side lost 2-0 to Eric Tinkler's men at Orlando Stadium on Saturday night.
Zamalek were defeated 1-0 at AC Leopards on Sunday, leaving the Egyptians level with Bucs on nine points at the top of Group B after four games.
Leopards remain in the hunt for a last-four berth with four points, while Sfaxien have been eliminated from the second-tier continental event.
“Orlando is Orlando,” Duarte is quoted as saying on the Buccaneers' website.
“They can be weaker in the local league or lose a game but in Africa they are something else.
“The winner of the CAF Confederation Cup will come from Orlando Pirates, Al-Ahly or Zamalek. Those are the best teams in Africa in my view.”
He added: “They are a big team here. Outside they are a very big and competitive team.
“We came here with little expectation to win the match. It was never going to be easy to play against Pirates at home, because as they showed us in Tunisia, that they would be an even tougher proposition in front of their home fans.”