BC Volgaburmash needed two late free throws from Olga Arteshina and then survived a nervous final 18 seconds to leave Spain with a 59-58 triumph at Ros Casares.
The win clinches first place in Group A for Volgaburmash, who finished with nine wins in 10 games - the same record as Hungarians MiZo Pecs.
Volgaburmash and Pecs split their two games, but the Russians had a superior points differential in those clashes.
In a game billed as EuroLeague Women champions Volgaburmash and Amaya Valdemoro against her old team Ros Casares, the Spaniards almost spoiled the return of their former player.
Noemi Bofill was fouled by Arteshina with 25 seconds to play and made both free throws to put Valencia in front at 58-57.
The Samara team went to the other end and Arteshina made both of her shots from the charity stripe, however, after a foul by Elisa Aguilar.
That set the stage for a tense finish.
Ros Casares, who had called a timeout to discuss their end-of-game strategy, inbounded the ball and got it into the hands of Brazilian international Janeth, who like Valdemoro once played for the Houston Comets in the WNBA.
Janeth has been one of the coolest customers in the women's game over the past decade but she missed a potential match-winning jump shot with four seconds to play and Maria Stepanova rebounded to secure the triumph.
Valdemoro, who joined the Russian team from Casares in the summer, had 12 points and six rebounds.
The home fans must have wondered if their team would ever wake up after a very nervy start saw the visitors storm out to a 14-1 lead with 4:26 left in the first quarter on Arteshina's three-pointer.
The Spaniards fought back, spreading the points around, and took a 23-22 lead with just under five minutes to play in the first half on a jump shot from Aguilar.
Casares built some momentum and led 31-24 after Aguilar's lay-up. Ann Wauters' basket accounted for the final points of the half, however, and left the hosts in front at 31-26.
Neither Casares nor Volgaburmash were able to take a grip on the game, and it eventually went to the wire.
Aguilar accounted for three consecutive baskets for Ros Casares, one of them a three, and gave them a 56-55 advantage with 2:00 to play.
Stepanova's lay-up put the visitors back in front with 0:34 to go, and after the lead changed hands twice more following Bofill's and Arteshina's free throws, Janeth Arcain had a chance to win at the end but missed.
Arteshina led Volgaburmash with 17 points while Aguilar scored 16 in defeat for Ros Casares.