Fenerbahce Istanbul shooting guard Romain Sato was detained overnight by Slovenian police on charges of trying to enter the country with a forged passport and missed his team's 81-75 win at Olimpija Ljubljana after he was released and sent home.
Elsewhere, holders Olympiakos Piraeus suffered a 98-72 drubbing at Anadolu Efes Istanbul and Real Madrid, the winners of eight titles in Europe's elite club basketball competition, were edged by Russia's Khimki 86-85 after missing a game-winning shot on the buzzer.
Weems, who plied his trade for the Toronto Raptors and Denver Nuggets before moving to Europe in 2011, poured in 30 points as CSKA rallied to overcome a four-point deficit in regulation and then in overtime in front of a fervent but well-behaved crowd in Belgrade.
While his team mates struggled, Weems scored from all angles as he slam-dunked, buried three-pointers and nailed off-balance shots to silence 7,000 home supporters, who watched the game alongside several hundred CSKA fans in an amicable atmosphere.
"This is the second-best league in the world and playing in the NBA has taught me how to perform in big games, keep my focus and deliver under pressure," the 26-year old told Reuters.
"We didn't give up after we fell behind, we made some big shots and I felt very confident after getting into the right rhythm from the start despite the fact the home crowd was so loud we couldn't really communicate out there on the court.
"This is the best crowd I've seen since I started playing in Europe and I wish our fans were like that. This was a tough game for a new-look CSKA team which still has to gel a little bit but our ambition remains to win the Euroleague after finishing as runners-up last season," he added.
Fenerbahce club officials were incensed with Sato's detention, saying the Central African Republic player had entered a plethora of European countries for Euroleague games with the same passport.
"As a result of this unjust behaviour towards our player and our team we want to repeat that the legal struggle launched yesterday is continuing," Fenerbahce said on their website.
"Under the influence of this traumatic incident, it was decided by the management that Sato, who spent the night sleepless, should return to our country without waiting for this evening's match."
But Slovenian police said it had established the 31-year old was travelling on a forged passport.
"At the border control policemen discovered that the biography page of his passport was forged which was later confirmed by the suitable technical equipment," the police said in a statement sent to Reuters.
Fenerbahce, who also lost American playmaker Lester Bo McCalebb to an ankle injury in the second half, responded with a furious finish to make it two wins out of two in the 24-team competition.
McCalebb, who was carried off the court by his team mates, still finished as the game's top scorer with 17 points, Croatia forward Bojan Bogdanovic added 14 while Omer Onan and Mike Batiste chipped in with 13 each, the latter also collecting four rebounds.
Real Madrid trailed by 18 points at Khimki on the outskirts of Moscow but nearly turned the match on its head as former Portland Trailblazers guard Rudy Fernandez saw his last-gasp floater spin in-and-out of the rim.
"We changed our mindset after the third quarter, made a comeback but we have to play for the entire 40 minutes and we didn't," Real coach Pablo Laso told the competition's official website.
"Khimki deserve respect, they are a very good team with great players and we knew from the very beginning it would be difficult."