Marc Wilmots Rode Duivels sit above Argentina, Spain and World Cup holders Germany as the No.1 national team in the world thanks to their massive improvement over the last few years.
Belgium made it to the quarter-finals of the 2014 World Cup, losing out on a final four spot to Argentina, and with the likes of Romelu Lukaku, Eden Hazard, Kevin De Bruyne and Thibaut Courtois in their ranks, they are looking to put in a solid display at Euro 2016.
However, ex-Ireland player Duff finds it difficult to accept how well the nation has been rated because Manchester United's struggling midfielder Fellaini is considered a key player for them.
"First in the world in the rankings? It doesn't sound right," he told Independent. "Especially when Fellaini is playing for them."
Duff's former national team take on Belgium in their Euro 2016 group and, having crashed out in embarrassing fashion with zero points in 2012, the former winger is hoping for a better showing this year.
"That summer coming home, you didn't want to leave home and come into town for a few beers," he added. "You just wanted to keep the head down. But that's what you do in football.
"I've had bitter disappointments. I relegated Newcastle with an own-goal. But you move on; it would have been nice to go out on a better note."