Holders Olympiakos Piraeus still have plenty of work to do after throwing away a seven-point lead in the final minute of a dramatic game against Anadolu Efes Istanbul who levelled the series 2-2 after grinding out a 74-73 home triumph.
Real Madrid, winners of eight Euroleagues but seeking their first trophy since 1995, completed a 3-0 sweep of Maccabi Tel Aviv on Tuesday while 2010 champions Barcelona and Panathinaikos are level at 2-2 after the Catalans won in Athens on Thursday.
The deciding games in the Olympiakos v Efes and Barcelona v Panathinaikos ties will be played next week and the winners will join Real and CSKA in the showpiece event at London's O2 Arena from May 10-12.
Olympiakos appeared to be cruising after hitting nine three-pointers in an effervescent first half against Efes. Forward Kostas Papanikolaou led the charge with 14 points including four long-range efforts from five attempts.
Efes, however, turned the contest on its head by scoring the last eight points of the game in the final 60 seconds.
Former Los Angeles Lakers guard Jordan Farmar put them within striking distance with a three-pointer and forward Jamon Lucas sent the home fans into raptures with an off-balance tip-in with his weaker left hand after beating two Olympiakos players to a rebound.
Late drama also looked to be on the cards in the Basque Country where Caja rallied from a 14-point fourth quarter deficit to pull back to 82-85 before Serbian playmaker Milos Teodosic sealed CSKA's win with an audacious long-range shot.
Former Toronto Raptors guard Sonny Weems, Russia forward Victor Khryapa and Serbia centre Nenad Krstic scored 19 points each for CSKA, Teodosic added 13 and his compatriot Vladimir Micov chipped in with 12.
CSKA play Olympiakos or Efes in the semi-finals while Real will lock horns with traditional rivals Barcelona or Panathinaikos.