Bata - Captain Prince Oniangue scored the only goal of the game as Congo Brazzaville claimed a surprise 1-0 win against Gabon on Wednesday to throw Group A wide open at the Africa Cup of Nations.
Oniangue, of French Ligue 1 club Reims, struck three minutes into the second half in Bata and Gabon, who had dominated the first half, could not find an equaliser.
The result leaves Claude Le Roy's Congo, appearing at the Cup of Nations for the first time since 2000, on top of the group and needing just a draw from their final game against Burkina Faso in Ebebiyin to clinch a place in the quarter-finals.
It also leaves Pierre-Emerick Aubameyang and Gabon, 2-0 winners against Burkina Faso on Saturday, still with work to do to secure their own place in the last eight of this year's finals.
Jorge Costa's side knew that a win here would take them through with a game to spare and they had a host of chances to take the lead in the first half.
The pressure started right from the off, with Frederic Bulot seeing a fourth-minute volley turned over the bar by Christoffer Mafoumbi in the Congo goal.
Malick Evouna, who netted in Gabon's opening game, was then denied before Andre Biyogo Poko sent a rising shot over the bar as the Panthers continued to exert their authority.
Levy Madinda fired across goal and wide of the far post after showing some lovely skill inside the box and skipper Aubameyang, Gabon's star man, saw a free-kick boxed away by Mafoumbi in stoppage time right at the end of the first half.
But just as Gabon profited from Burkina Faso's inability to convert their chances at the weekend, so they paid the price for profligate finishing on this occasion.
Oniangue, an under-rated midfielder in the French top flight, pounced to fire home from close range soon after the restart after the Gabon defence failed to clear a corner.
Soon after, Oniangue almost scored his and his side's second goal with a snap shot from the edge of the box that was blocked by Didier Ovono.
However, Gabon squandered a glorious chance to level the scores midway through the second half as Bulot, of English side Charlton Athletic, somehow sent the ball wide of the right post with the goal at his mercy.