The game was held at the Miejski Widzewa stadium in Lodz and ended with a score of 2:0 in favor of the Portuguese club.
Gilberto (9th minute) and Gonzalo Ramos (37th) scored for Benfica.
The second match between the teams will be held on August 23 in Lisbon.
The match starts at 22:00 Moscow time.
Earlier, Bodø-Glimt with Khaikin beat Dinamo Zagreb in the first match of the Champions League qualification final.