The Flying Eagles surrendered a two-goal lead to record a draw in the African Youth Championship qualifier played on.
The Nigeria U20s were initially ahead with two-goal lead in the first half of the encounter, Forward, Wasiu Alalade netted a brace in the first half.
However, Guinea Bissau ended strongly and will take great encouragement from their second-half showing which saw them draw the tie with two goals of their own inside six minutes.
Saturday’s outcome now makes the second leg delicately poised even though the Flying Eagles have the advantage of not losing away from home and getting two important away goals.
The second leg comes up next weekend at the UJ Esuene Stadium in Calabar.
Nigerian U20 coach Paul Aigbogun blamed his boys for losing concentration in the second period, but expressed sureness that his team will get the job done in the return leg.