Alaya Brigui gave the home team the lead on 16 minutes at the Stade Olympique in Mediterranean resort Sousse.
The clubs meet again on a date to be decided during the weekend of October 20-22 at the 87,000-capacity Borg el Arab Stadium outside Alexandria.
Last weekend, Ahly shocked Esperance 2-1 near Tunis to reach the last four of the elite African club competition after being held at home.
Etoile and Ahly were renewing a rivalry that brought them together in the two-legged 2005 and 2007 Champions League finals.
An Ahly team inspired by now-retired Mohamed Abou Trika comfortably won the first final only to be stunned 3-1 at home when losing the rematch.
The liveliness of the latest episode between them came after a dreary 0-0 first-leg draw between USM Alger of Algeria and Wydad Casablanca of Morocco in Algiers on Friday.
Belgium-born USM coach Paul Put complained about the Cameroonian referee, claiming his team were denied a penalty.
Wydad coach Hussein Amotta blamed the Stade du 5 Juillet pitch for the poor fare, with barely any clearcut scoring chances.
"The state of the pitch affected our performance," he told reporters. "To play our normal passing game, we require a good surface."
Ahly have been African champions a record eight times with Etoile and Wydad once each. The closest USM came was finishing runners-up two seasons ago.
A record $2.5 million (2.1 million euros) first prize is up for grabs this year plus a place in the FIFA Club World Cup tournament scheduled for the UAE during December.