Both sides were playing fellow unbeaten outfits with CSKA (4-0) defeating Panathinaikos (3-1) 78-76 in an overtime thriller in Group B, while Barca (4-0) outplayed Montepaschi Siena (3-1) 92-75 in Group D.
In front of 16,000 fans in Athens, CSKA grabbed a big victory against the defending champions despite their top scorer, Ramunas Siskauskas, only netting 14 points.
In Barcelona, Chuck Eidson was the go-to-guy as he scored 22 points, while Erazem Lorbek (17) and Boniface N'Dong (15) also chipped in with valuable contributions.
Elsewhere on a busy night of action , SLUC Nancy (2-2) moved into third place in Group A by defeating the previously unbeaten Spanish outfit Caja Laboral (3-1) 90-85.
In Group B, Zalgiris Kaunas (1-3) got their first win of the season as they beat Brose Baskets (1-3) 81-70.
Real Madrid (2-2) slipped to their second defeat in Group C as they were upset at Partizan Belgrade (2-2) 80-79 in overtime.
Maccabi Tel Aviv (3-1) top the group after a 77-72 road win at Anadolu Efes ; while in the same group EA7 Emporio Armani ( 2-2) also won away from home by beating Belgian outfit Spirou Basket 81-76.