Claude Le Roy's men beat Gabon
Gabon missed a chance to book their place in the Africa Cup of Nations knockout stages as Group A winners, losing 1-0 to Congo on Wednesday.
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Jorge Costa's men beat 2013 finalists Burkina Faso 2-0 in their tournament opener and would have moved into an unassailable lead at the summit with another three points in Bata with a win in the second round of games.
However, Prince Oniangue capitalised on some questionable Gabon defending to score the winner three minutes after half-time.
Neither side was able to take control in a tight first half, but Frederic Bulot was unlucky not to break the deadlock with a spectacular strike after five minutes.
Oniangue's goal gave Congo something to defend, although Gabon came agonisingly close to finding a leveller when Bulot saw a great chance hit the outside of the right-hand post.
The result sees Congo, who drew 1-1 with hosts Equatorial Guinea in the event's curtain-raiser, go top of Group A ahead of the last round of fixtures of the opening phase.
Both sides made just one change from their opening matches, with Gabon bringing Aaron Ondele in for Yrondu Musavu-King at the back, while Francis N'Ganga made way for Dimitri Magnokele in the Congo XI.
Gabon looked to be full of confidence after kicking off the tournament with a victory, and almost went ahead with a rasping effort in the fifth minute.
A deep cross from the left was met by a neat Malick Evouna flick, but Bulot's rifled shot from the edge of the penalty area was tipped over by Congo goalkeeper Christoffer Mafoumbi.
Congo's Arnold Bouka Moutou lashed a volley into the side-netting at the other end, before Evouna saw his effort trickle wide via a deflection after racing on to a throughball in the 19th minute.
Recent encounters between these sides have been tight affairs - Congo beat Gabon 1-0 at home during qualifying for the 2014 World Cup before a goalless draw in the return fixture - and this meeting was shaping up to be no different.
Levy Madinda had a glorious chance to hand Gabon a half-time lead shortly before the interval, but dragged his shot wide of the left-hand upright after showing great composure in the box, before Pierre-Emerick Aubameyang stung the palms of Mafoumbi from a free-kick.
However, it was Congo who eventually struck first, with Oniangue firing home after defenders Ondele and Bruno Ecuele Manga had got in each other's way while trying to head clear from a corner.
Oniangue went close to doubling his tally when he unleashed a volley from the edge of the box soon after, but Gabon keeper Didier Ovono punched clear.
Aubameyang headed wide as Gabon chased an equaliser, before Bulot sent a low effort against the post when he should have pulled his side level midway through the second period.
Gabon continued to apply the pressure as the match approached a conclusion, but the Congo back line held firm to leave Claude Le Roy's men a point clear at the summit heading into Sunday's closing group fixture against Burkina Faso.