The strike followed an opening blaze of intensity from the Nigerians that caught Tunisia cold.
Jamilu Collins' surge down the left wing ended with a cross that the Tunisia goalkeeper Moez Ben Cherifa failed to collect. As the ball bobbled between him and defender Yassine Meriah, Ighalo prodded into the net to poach his fifth goal of the tournament.
Nigeria continued to dominate possession but without the same incision and gradually Tunisia established a foothold in the match.
Igahalo was unable to add to his tally. The 30-year-old left the action just before half-time with a thigh injury. He was replaced by the coltish Victor Oshimen who has been tagged as Ighalo’s successor.
However it was another substitute who had the chance to make an impact.
Firas Chaouat came on for Taha Yassine Khenissi just before the pause. The 23-year-old CS Sfaxien striker spent the break going through his paces.
To no avail. Just after the restart he was clear on goal but he lifted his shot over the bar with only the Nigeria goalkeeper Francis Ozoho to beat.
“We’re very happy to have beaten a side that is just above us in the rankings,” said Nigeria boss Gernot Rohr. “We knew it was going to be difficult so we’re very satisfied.”
Rohr, 65, who is in the third year of a four year contract, said he would take some time before deciding whether he would see out his deal with the Nigerian federation.
Alain Giresse, his Tunisia counterpart, hailed his team’s run to fourth place as a step forward following successive quarter-final appearances in 2015 and 2017.
“Tunisia hasn’t been in the semi-finals since 2004 so they’ve been waiting a long time and to do it at a competition with 24 teams is a good result,” said the former France international.
“The Tunisia team is the youngest at the competition and that should give us a boost for the work that has to be done to qualify for the 2021 Cup of Nations and the 2022 World Cup.”