Eight-times champions Real Madrid produced a fine performance to record an 83-75 victory at Fenerbahce while Panathinaikos Athens, winners of six Euroleague titles, eased to an 85-67 triumph at Slovenians Olimpija Ljubljana.
Olympiakos looked set to bounce back from last week's 98-72 drubbing at Efes Istanbul after grinding out a 39-33 first-half lead against Zalgiris who last won the title in 1999.
But the Lithuanian side turned the match on its head after the break, outscoring the home team 46-22 in the second half as 30-year-old Croatian shooting guard Marko Popovic finished with a game-high 16 points.
Olympiakos entered the new campaign with a virtually unchanged outfit. Forward Georgios Printezis, however, conceded they were now a pale shadow of the team that stunned CSKA Moscow in last season's final.
"It's a big mistake to compare last season's team to the current one," Printezis told the competition's official website (www.euroleague.net).
"We have to work hard and find a solution. We are the defending champions but it doesn't mean anything because this is another season."
Olympiakos, who lost their trophy-laden Serbian coach Dusan Ivkovic during the off-season, could suffer another blow.
According to media reports American centre Joey Dorsey is unhappy with the team's roster and is contemplating a move to the NBA.
Friday's defeat left the Greek champions fifth in their section with one win and two defeats. Four teams from each of the four groups advance to the last 16.
"Olympiakos are considered favourites in every game after last season's triumph but this is not true, it's far more difficult to manage success than to manage a failure," said new coach Georgios Bartzokas who faces a tough task to steady the ship.
Chasing a record ninth title that would put them on a par with their soccer equivalent, Real Madrid looked worthy contenders against an expensively assembled Fenerbahce side who were roared on by their fervent fans in the Ulker Sports Arena.
Real, who last won the Spanish league in 2007 and the Euroleague in 1995, controlled the match from start to finish as 11 of their 12 players got on the scoresheet.
"It was a great and important road win for us against a very strong opponent. We played very well at both ends of the court and denied Fenerbahce a comeback although they kept trying to get back into the game," said Real coach Pablo Laso.
Fenerbahce forward Ilkan Karaman said: "We played hard but in this game playing hard wasn't enough because we didn't play smart and gave away too many offensive rebounds".
Panathinaikos, who saw their talented squad break up during the off-season as the top players moved to wealthier rivals, overran Olimpija thanks to 18 points from Lithuanian guard Jonas Maciulis and 14 by Croatian playmaker Roko Leni Ukic.
Elsewhere, 2010 champions Barcelona strolled to a 72-60 home win over Besiktas and CSKA never moved out of second gear in a 76-67 defeat of Germany's Bamberg.
Serbian forward Nemanja Bjelica registered 20 points to lift Spaniards Caja Laboral to a 95-85 victory in Milan.
Spanish teams made it two wins out of two against the Italians after Unicaja Malaga ground out a 91-89 overtime win at home to Montepaschi Siena with Earl Calloway scoring the last five points for the hosts in a frantic finale.
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