With several players and coach Mondher Kebaier absent following a Covid-19 outbreak in the squad, Tunisia secured an unlikely 1-0 victory thanks to Youssef Msakni’s strike.
The long-range, swerving effort two minutes into the start of the second half was reached by Maduka Okoye, but the Nigerian goalkeeper couldn’t prevent the shot from finding the back of the net.
Nigeria, the only team to win every game in the group stages, had substitute Alex Iwobi sent off midway through the second half after a video assistant referee (VAR) review upgraded the forward’s tackle from a yellow to a red card.
The Super Eagles continued to push for an equalizer and had late opportunities through Wilfred Ndidi and Umar Sadiq, while Tunisia’s Naïm Sliti also had an effort saved by Okoye on 75 minutes.
But the scoreline remained 1-0, meaning Tunisia — which won just one game in the group stages and only progressed to the last 16 as one of the best third-placed teams — faces Burkina Faso in the quarterfinals on Saturday.
The Eagles of Carthage are bidding to win their second AFCON title having previously triumphed in 2004.
Meanwhile, Burkina Faso also progressed to the final eight after beating Gabon on penalties.
Bertrand Traoré, who hit the bar from the penalty spot in the first half, gave his side the lead when he finished off a one-on-one, but Gabon equalized in stoppage time of the second half through Adama Guira’s own goal from a corner.
That was after Gabon’s Sidney Obissa had been sent off for a second yellow card.
In the shootout with the scores poised at 6-6, Lloyd Palun hit the bar for Gabon before Ismahila Ouédraogo converted to send Burkina Faso through to the quarters.