The Guinean team reached the interval already ahead, thanks to a goal by Dramane Nikiema, in the 36th minute, after a penalty kick, but this was not the only setback for Petro, who saw midfielder Joaquim Leitão sent off in the 44th minute, for accumulating yellow cards.
In the second half, Horoya confirmed the victory on the brink of the match, in the 85th minute, by striker Yakhouba Barry, which gives them, for now, the lead of group C, with three points.
The other two teams in the group, the South African Kaiser Chiefs and the Moroccan Wydad Casablanca, are yet to make their debut in the competition.
Meanwhile, for the first leg of the last qualifying round for the group stage of the Confederation Cup, also known as Nelson Mandela Cup, 1.º de Agosto, coached by the Portuguese Paulo Duarte, did not receive, this Saturday, in Luanda, Namungo, from Tanzania, because five of his players tested positive for covid-19 on arrival in the Angolan capital.
