Belgium substitute Divock Origi grabbed a winner two minutes from time against Russia to take his side into the last 16.
There was not much action in the first half as Alexander Kokorin missed the best chance of the half, heading wide from six yards.
Substitute Kevin Mirallas hit the post with Belgium’s first shot on target with seven minutes remaining but Origi turned in from Eden Hazard’s cross to take Marc Wilmots’ team into the knock-out phase.
Russia’s hopes of now qualifying into the last 16 now hinge on their final group match against Algeria.
Origi impressed when he came off the bench in the opening 2-1 victory over Algeria and once again the Lille forward did well when he replaced Romelu Lukaku who appeared far from happy with the decision early in the second half.
They may have six points from two matches, but Belgium who lost Arsenal defender Thomas Vermaelen to injury will have to up their game if they are to continue with their progress. Mertens was given a starting place and carried all of Belgium’s threat down the right flank in the first half.
The Napoli winger had an effort blocked after a fine run and, following Kevin de Bruyne’s surge forward, he attempted to feed Romelu Lukaku rather than shooting from a good position.
Mertens drove wide of goal after another impressive break but other than that Russia were barely troubled in a slow opening half.
They went close themselves when Viktor Fayzluin’s low effort was pushed away by Thibaut Courtois, who also made a scrambling stop from Maksim Kanunikov.
Kokorin should have opened the scoring just before the interval but he glanced a header wide at the far post from Denis Glushakov’s fine cross.
Glushakov thumped a wild effort off target from a narrow angle as the second half started at an even lower tempo with both sides appearing content with a draw.
Andrey Eschenko dragged an effort wide after a decent Russian move, then Everton winger Mirallas was denied by the foot of a post.
Hazard drove wide after breaking into the area but the Chelsea forward made a more decisive contribution in the 88th minute, breaking down the left flank and pulling back for Origi to score.