The quarter-final winner came 12 minutes into extra time in port city Bata when Balboa superbly curled a free-kick into the top corner of the net past goalkeeper Aymen Mathlouthi. The result was among the greatest upsets in the 58-year competition with Tunisia 96 places above Equatorial Guinea in the FIFA/Coca-Cola World Rankings.
Ahmed Akaichi put Tunisia ahead on 70 minutes with a close-range shot and Balboa levelled in stoppage time by converting a penalty. The reward for 28-1 pre-tournament long shots Equatorial Guinea is a semi-final with Ghana or Guinea in capital Malabo next Thursday. Earlier, Congo DR staged a stunning comeback in the first match of a Bata double-header to defeat Congo 4-2 and secure a last-four showdown with Algeria or Côte d'Ivoire.
Ovono dropped a corner and needed medical attention after being challenged for the loose ball by Tunisian defence marshal Aymen Abdennour. A wild swing at the ball by Wahbi Khazri saw the ball fly wide and a few minutes later his close-range shot was deflected for a corner.
The best Tunisian chance of the half came on the half-hour when Ferjani Sassi evaded several defenders only to nod an Akaichi cross wide. Equatorial Guinea finally troubled Aymen Mathlouthi six minutes before half-time and his weak punched clearance almost led to a goal. Balboa pounced on the loose ball just outside the box and his low, goal-bound shot was cleared by Syam Ben Youssef. Tunisia finished the half stronger with Akaichi delivering another good cross that captain Yassine Chikhaoui handled as he battled for possession.
Akaichi brought out the best in Ovono soon after half-time with a snap shot that the young goalkeeper tipped over. The goal that ended the stalemate stemmed from a midfield pass by Chikhaoui to Mohamed Yaakoubi on the right wing.
His cross was aimed at Akaichi, who got in front of Diosdado Mbele and steered the ball past Ovono to stun the crowd. But two minutes into stoppage time home supporters erupted in joy as Balboa sent Aymen Mathlouthi the wrong way from the spot.