Playing in their first finals since 2002, the Belgians struggled to justify the pre-tournament hype for much of their Group H game with Algeria in Belo Horizonte but scored twice in 10 second half minutes for a 2-1 victory. They had dug a hole for themselves when defender Jan Vertonghen needlessly tugged back Algeria's Sofiane Feghouli to concede a first half penalty. Feghouli converted to give the north Africans their first goal at a World Cup since 1986.
After taking a surprise lead, Algeria were able to sit back and soak up Belgian pressure on a hot day, while continuing to threaten on the break.
Belgian playmaker Eden Hazard, closely marked by the Algerian defence, found it hard to exert much influence and striker Romelu Lukaku was forced to drop deep into areas where he posed no danger.
Belgian coach Marc Wilmots stresses the impact that substitutes can have on a game and Fellaini and Mertens illustrated the point perfectly.
Mertens came off the bench at the start of the second half and immediately provided an attacking spark down the right, helping to take the pressure off Hazard. Wilmots sent on Fellaini after 65 minutes and pushed him up alongside teenage striker Divock Origi, who had replaced Lukaku.
Fellaini, who endured a wretched first season at Manchester United after a 27.5 million pound ($46 million) transfer from Everton, looked like the player of old. He levelled the score at 1-1 after 70 minutes, guiding a flicked header in off the underside of the crossbar to convert a fine centre from the hard-working Kevin de Bruyne.
Mindful of potentially tougher games to come against Russia and South Korea, Belgium picked up the pace as they sought all three points.
The winner came courtesy of a classy counter attack that showcased Belgium's strengths.
De Bruyne, named man of the match, won the ball well and swiftly fed Hazard, who made use of some rare space to pick out Mertens with a perfect pass which the midfielder smashed past goalkeeper Rais Mbolhi. Among the unused substitutes for Belgium was teenage Manchester United forward Adnan Januzaj, another player with the ability to turn a game against tiring defenders.
With such a talented squad, Belgium are harbouring hopes of matching the exploits of their 1986 team which reached the semi-finals before losing to Diego Mardadona's Argentina. This win showed they also have the character to progress in Brazil.
For Algeria, there was the consolation of that first World Cup goal for 28 years and the confidence they will take from giving the group favourites such a tough game.