Yussuf Poulsen scored the only goal in the second half to give Denmark a 1-0 win over Peru, who earlier missed a penalty in their Group C opener on Saturday.
Christian Cueva ballooned his penalty, which was awarded after the referee consulted VAR, over the bar just before halftime, the most glaring of several misses for the South American side, who were back in the World Cup finals after a gap of 36 years.
Tottenham playmaker Christian Eriksen had an otherwise quiet outing but provided the decisive moment when he threaded an immaculate pass through the defence for Poulsen to beat advancing Peru goalkeeper Pedro Gallese in the 59th minute.
With France firm favourites to top the group and already picking up their first points with a 2-1 win over Australia earlier on Saturday, both Peru and Denmark were desperate for victory to boost their chances of advancing to the playoffs.
Both teams arrived in Saransk with an identical 15-match unbeaten streak and it appeared to be a matter of time before a bright-looking Peru, egged on by massive support at the stadium, would score.
Denmark started by playing long balls directed at their wingers to take advantage of their superior height factor but mostly ended up playing catch up to Peru's slick short passing game for the opening half-hour.
Erisken was largely ineffective during this period but started having an impact on the game once the Danes began playing more passes.
Peru, who left out all-time top-scorer Paolo Guerrero from their starting lineup, were awarded a penalty after consultation with the video assistant referee for Poulsen's foul. Yet Cueva struck an awful penalty, leaving the red-and-white clad Peru fans distraught.
The South Americans continued to attack in the second half and nearly opened the scoring in the 57th minute but Andre Carrillo and Edison Flores missed chances in a goalmouth scramble.
Denmark made them pay two minutes later on the counter-attack as Eriksen laid the ball of for Poulsen to finish inside the near post.
Guerrero came on after the hour mark and almost scored an equaliser but his header flew straight into the hands of goalkeeper Kasper Schmeichel. The marksman also came close when his audacious backheel trickled past a helpless Schmeichel but fell inches wide of the post.
Schmeichel then stopped a point-black attempt from Jefferson Farfan, and Denmark also went close when Eriksen was played in but Pedro Gallese did well to save
Peru ultimately could not find an equaliser and Denmark joined France on three points atop the group.