Gabon were thwarted in their bid to become the first nation to reach the 2015 African Nations Cup quarter-finals as Congo pulled off a surprise 1-0 victory in Bata.
Jorge Costa's Panthers beat Burkina Faso in their Group A opener and would have sealed their progression to the knockout stages with a second win, but Congo had other ideas.
Prince Oniangue scored the only goal of the game following a defensive mix-up in the 48th minute and it proved enough to secure the Red Devils their first finals triumph since 1974.
Congo, who are coached by Claude Le Roy, move on to four points ahead of their final-round meeting with Burkina Faso while Gabon must regroup and plot the downfall of hosts Equatorial Guinea.
Gabon revealed their determination to reach the last eight in the opening exchanges as Frederic Bulot tested Congo goalkeeper Christopher Mafoumbi with a searing volley from the left flank.
In the 19th minute Malick Evouna fizzed a shot wide of the mark and Andre Biyogo Poko was not far behind him with a left-footed effort that flew over Mafoumbi's crossbar.
Congo did tighten their lines to keep Gabon at bay but Levy Madinda sneaked through in the minutes leading up to half-time and dragged the ball across the goalmouth and wide at the far post.
Pierre-Emerick Aubameyang – the Borussia Dortmund striker – stung Mafoumbi's palms with a free-kick in stoppage time yet Congo had done enough to maintain the stalemate heading into the break.
It came as some surprise, therefore, when they managed to open the scoring following the restart.
A corner from Arnold Bouka-Moutou was badly cleared in the 48th minute and Oniangue thought quickest to stab the ball home from six yards out.
The Reims defender soon forced Didier Ovono Ebang into a punching save before Gabon bit back on the hour mark, Madinda volleying with force but over the bar.
Aaron Appinda Ngoye and Bulot were next to try their luck, to no avail, while Bouka-Moutou responded for Congo by heading over in the 76th minute.
Madinda twice worried Mafoumbi inside the last 10 minutes, first shooting past a post and then finding his gloves with a swerving effort from distance, but Congo had done enough to win all three points.
Meanwhile, Burkina Faso were left clinging to their African Nations Cup future after a frustrating goalless draw with hosts Equatorial Guinea.
Knowing defeat would see them eliminated, the 2013 runners-up pressed forward throughout and hit the woodwork twice in the first half through Alain Traore.
But their end product was increasingly lacking as the match went on and they will need other results to go their way if they are to qualify.
Burkina Faso threatened first when Alain Traore's free-kick curled around the wall but bounced against the left post.
They had an alarm when goalkeeper Germain Sanou was wrongly penalised for handling outside his area, but they broke from the free-kick and Jonathan Pitroipa was brought down by last man Diosdado Mbele in a wide position.
Only a yellow card was shown – enough to see Mbele suspended for the final group game against Ghana – and after Randy Iyanga conceded a second free-kick Aristide Bance's fierce drive was parried away by keeper Felipe Ovono.
Sanou had to be alert to clear Bakary Kone's risky back-pass and he then made a solid near-post save from Armando Bohale, who had no support in the box.
Alain Traore's volley was fumbled against the near post by an uncertain Ovono and nearly fell to Bance before Bertrand Traore brilliantly beat Iyanga and fired in a shot which was turned away.
Ivan Zarandona kept Equatorial Guinea on terms, first with a vital interception with Bance closing in and then by clearing Bertrand Traore's low goalbound shot.
Emilio Nsue had a shot blocked for the hosts but Alain Traore then wasted some good work by younger brother Bertrand with a wild finish.
Bance's header across goal early in the second half almost fell to Pitroipa while at the other end, Nsue raced onto a through-ball but could not control and Sanou snuffed out the chance.
Alain Traore sent a sidefoot volley high and wide and Bance's shot took an awkward bounce and troubled Ovono.
Equatorial Guinea broke from their opponents' corner but Kike Boula curled high and wide.
Rui Da Gracia blocked another powerful Bance drive and Pitroipa flashed a shot just over before Alain Traore sidefooted wide.
Sanou had to flap a corner away from 6ft 6in Equatorial Guinea substitute Raul Fabiani Bosio and Burkina Faso appeared to have run out of ideas at the other end as they failed to find a breakthrough.
