Ex- African Footballer of the Year Victor Osimhen helped Nigeria establish a 3-0 lead, but they were compelled to defend resolutely for a narrow win.
Nigeria weathered a stunning comeback attempt from Tunisia to advance to the last 16 of the Afcon tournament taking place in the host nation.
Jose Peseiro's side seemed to be in complete control in their pool encounter in the Moroccan city, enjoying a three-goal cushion with only a quarter of an hour remaining thanks to goals from Victor Osimhen, Wilfred Ndidi and Ademola Lookman.
Yet, Montassar Talbi pulled one back with a powerful header from a Manchester United midfielder set-piece, igniting hopes of a turnaround.
The drama intensified when Tunisia were given a spot-kick after a video assistant referee review spotted a handling offense by the Nigerian defender. Ali Abdi calmly slotted home in the dying stages to set up a nail-biting finale.
Tunisia came agonizingly close from a stunning leveler in stoppage time, with their skipper directing a opportunity narrowly wide before a substitute sent a half-volley wide of the upright.
Securing First Place
This result means that Nigeria, winners of the tournament on three previous occasions, move to six points and are assured top spot in their pool with a match left to be contested.
For the round of 16, they will face a best third-place team from one of the other preliminary groups.
Meanwhile, Tunisia stay on 3 points, with the East African teams locked on one point after playing out a 1-1 draw in the day's other fixture.
The concluding pool fixtures will see the group leaders stay in Fes to play the Cranes on the next matchday, while Tunisia return to the capital to face the Taifa Stars.
An Anxious Conclusion
The Tunisian defender smashed home from the penalty spot to give his team hope of earning a point.
The Super Eagles, runners-up in the previous edition, are the second team after Egypt to reach the knockout stage, but their manager and fans will undoubtedly be breathing a sigh of relief.
What looked like set to be a comfortable final quarter transformed into a nerve-wracking affair.
The prolific striker had a goal disallowed for offside before breaking the deadlock right before half-time, expertly guiding a glancing effort into the bottom corner from an Atalanta winger delivery.
The lead was extended early in the second half when Wilfred Ndidi climbed above everyone to thump in a powerful nod from a Lookman corner.
The number 9 then set up Lookman for the third goal, before the defender to steer a powerful header past the Nigerian shot-stopper to begin the comeback.
The key moment arrived when a high ball struck the forearm of Bright Osayi-Samuel, with referee Boubou Traore pointing to the spot after consulting the VAR monitor.
Although the defender's confident conversion, the 2004 champions ultimately came up just short of pulling off a stirring comeback.
Tunisia's destiny is still in their control; a point against Tunisia will be sufficient to see them through, and their coach will be eager to avoid a recurrence of the 2013 group-stage exit that led to his previous resignation.
