Spain will start their World Cup campaign against Portugal on June 15 in Sochi and Iniesta is wary of facing the Euro 2016 champions.
The midfielder has faced Ronaldo many times in El Clasico, knowing exactly what the five-time Ballon d’Or winner has to offer for club and country.
“Portugal is a team that takes a lot to defeat and we will have to perform at the highest level. In a World Cup, any small thing can knock you out,” Iniesta told Cadena Cope.
“Cristiano is a fundamental player for any team he is in and for Portugal as well.”
Iniesta also admitted that it may be the right time to end his international football, especially as he will be playing in Japan from next season.
“I will never close one hundred percent the door of the national team after the World Cup, but everything indicates that this will be the case,” he admitted.