Cameroon raced into a two-goal lead through Mbia and Geremi before Chaouki Ben Saada fired Tunisia back into the game.
Yacini Chikhaoui sent the match into extra-time before Mbia scored again to secure a semi-final date with hosts Ghana.
Geremi Leads From The Front!
Defending champions Egypt used all their African Nations Cup experience to seal their place in the semi-finals with a hard-fought victory over Angola.
Hosni Abd-Rabou opened the scoring for the Pharaohs midway through the first half only for Manucho Goncalves to equalise with a spectacular effort.
But Amr Zaki had the final say, making the most of sloppy defending from the Palancas Negras to book a last-four meeting with Ivory Coast on Thursday.