Belinda Snell led the way with 21 points, six rebounds and four assists.
The home side got off to a fast start thanks to the unlikely hero.
Snell, her team's sixth leading scorer coming into the game (7.8 ppg), turned in 13 first quarter points using every inch of the court.
Six of those points came from beyond the three-point line, four from inside it and three from the charity stripe.
Avenida meanwhile relied on their star as Sancho Lyttle totaled seven points in the first 10 minutes, keeping Ros Casares within striking distance.
The home team started slowly in the second quarter and Avenida pounced, with a Lyttle lay-up making it 23-18 at the 5:40 mark.
The pendulum would just as quickly swing in the opposite direction.
With just over five minutes left Halcon coach Jordi Sanchez received a technical foul and the girls from Valencia capitalized.
Elisa Aguilar hit the two free throws and Laia Palau buried a three-pointer to push the Ros Casares lead back to 10, 28-18.
Amaya Veldemoro would eventually make it 13 on a three-pointer with 3:36 remaining, but Avenida would fight back and found themselves down a manageable 10 points at the break.
The second half opened much as the first had, with Belinda Snell.
The 1.80m small forward opened the scoring with three points the old fashioned way pushing her team's advantage back to 13.
Erica De Souza, Jana Vesela and Laia Palau would all contribute to an 8-4 Ros Casares run that saw the lead balloon to 17, and it appeared the rout was on.
But Avenida had one more push left in them.
Le'Coe Willingham led the comeback with a pair of buckets down low, followed by two free throws that cut her team's deficit to 10, 50-40.
But that is as close as they would come as Willingham, a respectable 41% three-point shooter, missed a pair of triples, both of which Ros Casares capitalized on to push their lead back to 15, and the rest was academic.
Willingham led the way for Avenida in defeat with 17 points and eight rebounds.
Erika De Souza, Ciudad Ros Casares player:
"I'm sure that in Salamanca the game will not be like this first one. We must be very tough to win there."
Jordi Fernández, Halcón Avenida coach:
"It's not easy to win in Valencia, but we're going to compete [better] in Salamanca. We want to win in Salamanca and to bring the series back to Valencia for the decider."