CSKA Moscow, six-time winners of Europe's premier club competition, impressed with a 90-71 home rout of Anadolu Efes Istanbul on Friday and their less heralded neighbours Khimki, based on the outskirts of Russia's capital, sprung an 80-75 upset at Besiktas Istanbul.
Panathinaikos Athens edged Lithuanians Zalgiris Kaunas 67-66 in a battle of former champions while Italians Montepaschi Siena produced an effervescent performance in a 79-69 home win over Maccabi Tel Aviv, the winners of four Euroleague titles.
Caja Laboral Vitoria were staring at an early exit after losing six of their opening seven games in the preliminary group stage of the fiercely competitive 24-team competition earlier in the season, two of them to holders Olympiakos Piraeus.
Zalgiris, CSKA advance into top 16
But in their opening match of the top-16 stage, featuring two pools of eight teams playing each other home and away on a round-robin basis, the Spaniards got sweet revenge with an 82-74 victory against the defending champions after overcoming an 11-point first-half deficit.
Polish centre Maciej Lampe engineered Caja's fourth win on the trot with a career-high 25 points and American shooting guard Brad Oleson added 15 as the Basque outfit powered ahead after winning the third quarter 27-11 in the cauldron of their Fernando Buesa Arena.
"Caja is a completely different team from the one in the regular season as they now have the confidence and the chemistry while our defence was not what it was supposed to be," Olympiakos coach Georgios Bartzokas told the competition's official website (www.euroleague.net).
His counterpart Zan Tabak said: "It has been very difficult to make the players believe they can win after they lost twice to Olympiakos already but we prepared mentally very well for this game."
Eight-time winners Real Madrid strolled to a 77-63 success at Alba Berlin on the back of 13 points apiece by Rudy Fernandez, Nikola Mirotic and Jaycee Carroll, while 2010 champions Barcelona outclassed Fenerbahce Istanbul 100-78 in their Blaugrana Arena.
Unicaja Malaga were perilously close to spoiling the Spanish party after falling behind 15-0 in the opening few minutes of their home tie against rank outsiders Brose Baskets Bamberg, who squeezed into the top 16 with a 3-7 record thanks to a last-gasp win over Partizan Belgrade.
But the Germans were undone by a patient fightback from Malaga, who turned the match with a strong final quarter and won 85-82 as shooting guard Marcus Williams buried a pair of crucial three-pointers to finish with a game-high 19 points.
"Every game in the top 16 will be very tough and we have to stay focused," Malaga's Croatian coach Jasmin Repesa said. "We don't have stars so it's important to play as a team, with energy and chemistry."
CSKA nailed their opening six three-pointers and Efes matched them all the way in an enthralling first half but last season's runners-up romped away after the break, when former Toronto Raptors forward Sonny Weems scored 18 of his 22 points in a what was almost a one-man show.
Serbia centre Nenad Krstic also had 22 points while three other CSKA players chipped in with 11 each as Efes, whose playmaker Jordan Farmar was a lone beacon of light for his team with a game-high 24 points, ran out of steam after heading into the locker room on level terms at halftime.
"My team were smart, unselfish and disciplined enough to find Weems and Krstic, who stepped up tonight," said CSKA coach Ettore Messina. "Besides (Aaron) Jackson did an excellent defensive job on Farmar in the second half."
A fast and flowing Barcelona blew away Fenerbahce after collecting 16 rebounds and 11 assists more than the Turkish side. They also had six players in double scoring digits led by Brazil guard Marcelinho Huertas and Juan Carlos Navarro with 14 points each.
Tomas Ress inspired Siena against Maccabi with 18 points, hitting six of his eight shots from the field and all three from the foul line, while former Miami Heat and Los Angeles Lakers forward Jason Kapono led Panathinaikos with 16 on his debut in Europe.
The top four teams from each pool will qualify for the best-of-five quarter-final playoff series, whose winners will advance to the May 10-12 Final Four in London.
Maccabi, Zalgiris lose unbeaten records
Olympiakos hand Nocioni sour Euroleague return