The visitors stormed into an early lead, with Caja Laboral trailing 22-13 after the opening quarter and Olympiacos then keeping up the pressure to lead 66-60 going into the final period.
The home side ramped up the pressure in the final quarter, however, inspired by Mirza Teletovic (pictured) with 20 points and five rebounds. They eventually caught and then overhauled their Greek opponents to lead 79-74 with less than two minutes left.
Olympiacos fought back to square the scores, yet Pablo Prigioni's two-pointer with seven seconds left gave victory to Caja Laboral - though not before Martynas Gecevicius missed a chance to force overtime just as the buzzer sounded.
In Croatia, KK Zagreb were humiliated in front of their home fans as CSKA Moscow destroyed them 89-47, with both Nenad Krstic (15 points and five rebounds) and Andrei Kirilenko (14 and seven) shining for the in-form visitors.
Montepaschi Siena also enjoyed a healthy win as they buried Galatasaray 103-77 at the Palaestra Arena, with Rimantas Kaukenas scoring 24 points on a golden night when he only missed with one of his 15 attempts on the basket.
Danny Green was the hero for Slovenian side Union Olimpija Ljubljana , scoring 23 points as his side beat Asseco Prokom Gdynia 70-62 at the Stozice Arena.