Belgium thrashed underdogs Tunisia as the Red Devils' stars Eden Hazard and Romelu Lukaku scored two goals apiece and Belgium sub Michy Batshuayi produced one goal in the stoppage time.
Tunisia's goals were scored by defender Dylan Bronn and forward Wahbi Khazri.
Chelsea star Hazard was named man of the match.
Their triumph in Moscow put Belgium in a strong position on top of Group G with six points, and Tunisia in last place with no points.
Two other teams in the group, England and Panama, will meet Sunday in Nizhny Novgorod in a match kicking off at 1200GMT.
In Group G standings, England follow Belgium with three points, and Panama are in third place with no points.