Sundowns rested their regulars Denis Onyango, Themba Zwane, Gaston Sirino, Rivaldo Coetzee, Andile Jali, Mosa Lebusa and Rushine De Reuck.
The Downs technical team gave a run to the likes of George Maluleka, Sibusiso Vilakazi, Aubrey Modiba and Khuliso Mudau.
However, the Brazilians, who had already qualified going into the match, conceded two first half goals for their first defeat at home in four years.
A resurgent CR Belouizdad opened the scoring in the 29th minute when a low pass from goalkeeper Ricardo Goss fell to Amir Sayoud, who rounded the former Bidvest Wits goalie.
They got their second goal at the stroke of half time when Ahmed Gasmi headed home after a pinpoint cross from Zinelaabidine Boulakhoua.
An unconvincing Masandawana battled to get back into the game. Meanwhile, TP Mazembe beat Sudan’s Al Hilal 2-1, finishing third in the group.
By Mthokozisi Dube