Guinea Bissau are now top of Group A of the 2023 Africa Cup of Nations qualifying series.
This follows their 1-0 victory over Sao Tome & Principe on Wednesday in the Lusophone derby.
Sao Tome ceded home advantage to Guinea Bissau because their own stadium has been deemed unfit for use in international competition.
The win takes them up to 10 points after five games with a one-point lead over the Super Eagles.
Zinho Gano scored a 55th-minute winner to hand Guinea Bissau all three points.
They will confirm a place in their fourth successive AFCON if Sierra Leone fail to beat Nigeria in the other group game on Sunday.