Lubango - Gabon remained in control of Group D with a goalless draw against Tunisia on Sunday.
Alain Giresse's side will confirm their passage to the last eight if they beat Zambia in their final game, to be played in Benguela on Thursday.
Tunisia, meanwhile, must beat Cameroon and hope other results go their way if they are to make it through.
As in their opening game, a 1-0 win over Cameroon, Gabon were unspectacular but sound, while goalkeeper Didier Ovono confirmed his growing reputation with another assured display.
Gabon began edgily, and were twice nearly undone early on by defensive errors.
First Hocine Ragued was left unmarked at the back post from a corner, but put his header wide, and then an underhit backpass from Bruno Manga laid in Amine Chermiti, who sliced his finish badly wide.
Gabon gradually gained a foothold in the game, but their best chance of the first half came during their period of uncertainty, an unmarked Eric Mouloungui seeing his plunging header flash just wide.
The Azingo Nationale took control after half-time, and forced a series of corners and dead-balls around the box, without ever really looking as if they would have the craft to break Tunisia down.
Their only real opportunity came from a bizarre error from Tunisia goalkeeper Aymen Mathlouthi, who misjudged the flight of an overhit cross and skewed his punch only just over his own crossbar.
Cameroon play Zambia in the day's second Group D game later Sunday.