ISTANBUL (Euroleague) - Teams can go from the penthouse to the outhouse in a very short space of time and Fenerbahce are in danger of doing just that.

After beginning the Euroleague Top 16 with three straight wins, Neven Spahija's side has slumped to back-to-back defeats to leave their quarter-final hopes hanging by a thread.

Following last week's overtime setback at Zalgiris, Fener returned home to the Sinan Erdem Dome and on Thursday night slumped to an 80-65 home setback against Olympiacos - the same team they beat in Athens by 14 points at the start of this round of the competition.

Fener were outscored 26-10 in the last quarter.

Olympiacos, who got big games from Zoran Erceg (19 points), Loukas Mavrokefalides (18pts), Rasho Nesterovic (16pts, 9rbs) and Theo Papaloukas (9pts, 8ass, 4rbs, 3st), sealed first place with the 15-point triumph.

Turkey international Omer Onan (pictured), who had two games of 22 or more points in Fener's first four Top 16 games, best symbolised his team's struggles as he was held to just two points.

Fener must now travel to Spain and avoid losing to Power Electronics Valencia next week by three or more points or else they will miss out on the quarter-finals.

Valencia, who fell 75-73 at Fener last month, stayed alive in the competition with an 80-74 victory at Zalgiris.

Adding spice to the encounter is the fact that Spahija led Valencia to the 2010 Eurocup but then left last summer.

Spahija was very unhappy with his team's concentration in the latter stages of the game against Olympiacos.

"We had a nice system on defense and offense but we lost the game in 14 minutes," he said.

"When Olympiacos took the momentum of the game, then my players gave up defending."

In Lithuania, the big development for Valencia was coach Svetislav Pesic not only activating Jeremy Richardson following a metatarsal injury suffered on Monday by Victor Claver that has ruled the Spain international out for two months, but starting Richardson and getting 17 points from the American.

Nando De Colo also poured in 17, Rafa Martinez scored 14 and Robertas Javtokas 13.

Valencia point guard Omar Cook, meanwhile, had 12 assists to equal the Top 16 record that had been shared by Papaloukas and Vrbica Stefanov.

Claver watched the events unfold at home in Valencia and gave his reaction with a Tweet: "GREAT! At home you suffer much more than playing, although I've been pretty calm ... Now we just have to see what Fenerbahce and Olympiacos do."

Defending champions Regal Barcelona needed overtime but won 92-85 at Maccabi Tel Aviv to clinch first place in Group F at 5-0 with the Israeli club second at 3-2.

Former Maccabi star Terence Morris had 19 points to lead Barca, with Juan Carlos Navarro contributing 17.

Morris was five of nine from behind the arc and Navarro four of eight.

Caja Laboral created a three-team logjam in Group E by winning 78-63 at home over Unicaja Malaga.

The Vitoria side is now 3-2, the same record as Panathinaikos and Lietuvos Rytas.

Caja Laboral play at Lietuvos Rytas next week while Pana must travel to Spain to face Unicaja Malaga (1-4), who have no chance of progressing.

If Lietuvos Rytas win that game, they will advance but Caja Laboral would still progress with a defeat if Pana were to fall at Unicaja.

Montepaschi Siena have bounced back from an 0-2 start in Group G of the Top 16 to seal a last-eight spot.

On Thursday, they won at home over Efes Pilsen, 88-76, after getting 15 points from Milovan Rakovic and 14 from Rimantas Kaukenas.

Madrid improved to 5-0 and sealed first place ahead of Montepaschi in Group G with a 61-56 victory at Partizan Belgrade (0-5).

Ante Tomic poured in 21 points for Madrid, who held Partizan to just one of 20 shooting from behind the arc.

Jan Vesely of the Czech Republic, recently named FIBA Europe Young Player of the Year, had 15 points to lead Partizan.

FIBA