Eight-time winners Real Madrid could face an early exit from basketball's Euroleague after a 93-69 rout by fellow Spaniards Bilbao.
Real Madrid, who played in last year's final, were upset by their unfancied compatriots whose 9,000 fans in their jam-packed city arena roared them on to a decisive 34-14 lead in the first quarter.
Both teams have two wins and two defeats but Euroleague debutants Bilbao will beat Real to the runners-up spot and a last-eight berth if they finish level on points, having lost by a smaller margin in Madrid.
Former winners Barcelona and Maccabi Tel Aviv, as well as holders Panathinaikos Athens, enjoyed victories, while Galatasaray ended CSKA Moscow's perfect record with a 68-64 victory over the winners of six Euroleague titles.
Aiming for their third title in Europe's premier club competition, 2010 winners Barcelona ground out a 67-58 win at Lithuanians Zalgiris Kaunas to stay on course to reach the quarter-finals.
They have a 4-0 record in the top-16 Group H, ahead of Maccabi who are tied with Italians Bennet Cantu on two wins and two defeats each but ahead of them with a better head-to-head record after beating them 75-60 at home.