Super Eagles of Nigeria are the first team to qualify for the round of 16 at the ongoing African Cup of Nations in Egypt.
Three-time winners Nigeria scored the only goal in the 73rd minute when Moses Simon’s right-wing corner was headed in at the near post by Kenneth Omeruo.
They now have six points from their two games after a 1-0 win over Burundi in their first match on Saturday.
The top two nations in each of the six groups advance, along with the four best third-placed countries.
Nigeria dropped captain John Mikel Obi, who played for Middlesbrough in 2018-19, but their side did include Brighton defender Leon Balogun, Stoke midfielder Peter Etebo, Leicester midfielder Wilfried Ndidi and Arsenal forward Alex Iwobi.
Iwobi had one of the best chances when it was goalless, with a curled effort from just outside the penalty area.
But Guinea goalkeeper Ibrahim Kone produced a spectacular one-handed save to push it over the bar.
Guinea’s Naby Keita, who was making his first start for club or country in about seven weeks, received some strong challenges in order to be kept quiet until he was substituted.
He has replaced just before a Nigeria corner kick and Guinea went behind instantly, through Omeruo’s header to Moses’ right flank corner kick.
The group’s next matches are on Sunday, with Nigeria needing only a draw against Madagascar to win the group.