Six-time winners CSKA, who last won the title in 2008, blew away the competition's debutants Gescrap Bilbao in the opening two games in Moscow while Barcelona followed suit with a pair of home wins against Russians Unics Kazan.
Holders Panathinaikos Athens are tied 1-1 against last season's runners-up Maccabi Tel Aviv, as are their bitter city rivals Olympiakos with Italians Siena.
Former Utah Jazz forward Andrei Kirilenko scored a game-high 20 points and Serbian guard Milos Teodosic added 18 as CSKA beat Bilbao 79-61 on Friday following Wednesday's 98-71 rout.
But rather than savour the win which put CSKA on the verge of the season-ending tournament in Istanbul May 11-13, the team's Lithuanian coach Jonas Kazlauskas was fuming about what he called Bilbao's spoiling tactics.
"The opponent chose the only way that could have led to success and it was not basketball, it was something else," Kazlauskas said.
"There were so many provocations and unsportsmanlike fouls that we lost count and although it is difficult to deal with, I am thankful to Bilbao for this kind of physical game because we know what to expect in future.
Having eased to a 78-66 defeat of Unics in the first game, Barcelona found the Russians a much tougher prospect in the second and only an inspired performance by the Euroleague's all-time top scorer Juan Carlos Navarro prevented an upset.
Spain's talismanic playmaker poured in 18 points, including a treble of three-pointers, to give the 2010 winners a 2-0 lead before the series moves to Kazan next week.
"We lost Game 2 of the playoffs in each of the last two seasons and we were therefore uncomfortable in offence but making 23 of 25 free throws was the key," said Navarro.
Barcelona coach Xavier Pascual added: "We are happy to get our second win which justifies the good work we did all season.
"We fought hard to win our regular season group to get a better chance in the Top 16 and then we won our Top 16 group to get home-court advantage in the playoffs.
Panathinaikos appeared set to follow in the footsteps of CSKA and Barcelona but now have a mountain to climb in Maccabi's hostile arena after losing the second game 94-92 in overtime, having cruised to a 93-73 success in the first.
Forward David Blu nailed a pair of three-pointers in the last minute to silence the raucous home crowd in Athens and finish the enthralling rollercoaster with 16 points, one less than American shooting guard Keith Langford.
"It was a great, unbelievable match and the spectators saw tonight everything they would expect to see in a basketball game: great players, big shots, comebacks, ups and downs," said Maccabi coach David Blatt.
Olympiakos also spurned an excellent chance to take a 2-0 lead against Siena, throwing away their last possession in an 81-80 defeat on Friday after winning the opening game 82-75.
Playmaker Lester Bo McCalebb led Siena, looking for their second successive Final Four appearance, with 21 points while Serbian point guard Igor Rakocevic chipped in with 13.