The hosts have never before reached this stage of the competition, and their prospects of reaching a maiden final were improved by Koueme's second-half strike.
Ivorian midfielder Kouame struck 12 minutes after the interval to give Sewe a slender lead to take to the Democratic Republic of Congo for Saturday's return leg.
Victory continued Sewe's impressive recent home form, with Rigo Gervais' men now having won each of their last four matches at Stade Auguste Denise in all competitions, without conceding a goal.
They will have their work cut out in the second leg against a Leopards outfit that thrashed ASEC 4-1 in their last home fixture, and the 2012 champions will be looking for a similarly strong performance on Saturday.
The pair are battling for a place in the final against Al Ahly or Coton Sport, with the Egyptian side leading 1-0 from the first leg in Cameroon.
