Former Continent's Best Player of the Year the Napoli star helped his team build a 3-0 lead, but they were compelled to hold on for a hard-fought victory.
The three-time champions weathered a dramatic comeback attempt from Tunisia to progress to the last 16 of the 2025 Africa Cup of Nations being held in Morocco.
The Super Eagles seemed to be in complete control in their Group C clash in the Moroccan city, holding a 3-0 cushion with only a quarter of an hour left courtesy of strikes from their attacking trio.
Yet, a Tunisian defender reduced the deficit with a close-range finish from a Hannibal Mejbri set-piece, igniting hopes of a recovery.
The tension escalated when the North Africans were given a spot-kick after a video assistant referee check spotted a handling offense by Bright Osayi-Samuel. The left-back calmly slotted home in the dying stages to set up a frantic conclusion.
The Carthage Eagles were inches away from a stunning equalizer in stoppage time, with their skipper directing a opportunity just past the post before Ismael Gharbi sent a half-volley wide of the goal frame.
The victory means that Nigeria, winners of the competition on 3 previous occasions, move to six points and are assured first place in Group C with a match left to be contested.
In the next round, they will face a best third-place team from either Group A, B or F.
Meanwhile, Tunisia remain on 3 group points, with the East African teams locked on a single point each after registering a 1-1 stalemate in the day's other fixture.
The final pool fixtures will see Nigeria stay in Fes to take on Uganda on Tuesday, while the Eagles of Carthage travel back to Rabat to face the Taifa Stars.
The Tunisian defender smashed home from the penalty spot to offer Tunisia a glimmer of hope of snatching a draw.
Nigeria, finalists in the previous tournament, are the next nation after the Pharaohs to qualify for the next phase, but coach Eric Chelle and fans will certainly be feeling relieved.
What looked like set to be a straightforward final quarter transformed into a tense affair.
The prolific striker had a effort ruled out for offside before opening the scoring right before the interval, precisely placing a glancing effort into the far post from an Atalanta winger cross.
The lead was doubled early in the second half when the Leicester City midfielder rose highest to thump in a header from a set-piece corner.
Osimhen then set up his teammate for the seemingly decisive goal, before the defender to direct a header past goalkeeper Stanley Nwabali to initiate the comeback.
The key incident came when a looping cross hit the arm of Bright Osayi-Samuel, with referee Boubou Traore awarding a penalty after consulting the VAR monitor.
Despite the defender's confident conversion, the 2004 champions in the end came up just short of pulling off a remarkable comeback.
Their fate is still in their control; a point against Tunisia will be enough to secure progression, and manager Sami Trabelsi will be keen to prevent a repeat of the 2013 group-stage exit that led to his departure.
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