Ahmed Akaichi scored after 70 minutes to earn Etoile a 1-0 away win over Al Ahly Tripoli in a Group B match switched from Libya to Tunis for security reasons.
Should FUS defeat fellow Moroccans Kawkab Marrakech, Etoile will clinch a last-four place by winning at home next month in the penultimate mini-league round.
Abbas Mohammed bagged a brace as Medeama overcame Young Africans of Tanzania 3-1 in a Group A clash played on a cut-up pitch in Sekondi.
Daniel Amoah also scored for the Ghanaians, whose winning margin would have been wider had Malik Akowuah not had an early penalty kick blocked.
Simon Msuva converted a penalty for Young Africans, who are virtually out of running in the second-tier competition with just one point from four matches.
Medeama stayed third, behind Mouloudia Bejaia on goal difference, but those placings could be reversed Wednesday if the Algerians lose to leaders TP Mazembe in the Democratic Republic of Congo.