Gambia caused something of an upset just by getting out of their group, as they finished second to Mali, but their campaign shows no sign of stopping after it saw off its West African rival on Tuesday (AEDT).
A tetchy first half had few highlights but both sides did eventually threaten just before the break, as Guinea's Mohamed Bayo was denied at point-blank range by goalkeeper Baboucarr Gaye and Ebrima Darboe shot agonisingly wide at the other end.
The breakthrough arrived 19 minutes from time – Musa Barrow profiting from a fortunate ricochet before bursting into the box and coolly slotting home.
Guinea briefly thought they had levelled soon after, only for Ibrahima Sory Conte II to be flagged offside – he was then caught in the face accidentally as Yusupha Njie was somewhat harshly sent off for a second booking.
The Syli Nationale piled pressure on in the latter stages, hitting the frame of the goal twice in quick succession, but Conte then received his marching orders for a similar offence to Njie and Gambia held out for a famous win that will in all likelihood see them face hosts Cameroon in the last eight.