Mareks Jurevicus erupted for 29 points for Ventspils, including 13 made free throws from 15 attempts, and also corralled nine rebounds.
The Latvians improved to 2-3 with the victory and still have a chance of overtaking Cajasol Sevilla (2-2), who were in action later on against Group A leaders Belgacom Liege Basket (4-0).
Ventspils opened up a nine-point advantage late in the third quarter but Energa Czarni clawed their way back early in the fourth.
Demetric Bennett hit back-to-back baskets and his Energa teammate Marcin Sroka stroked a three-pointer to make it 65-63.
Janis Strelnieks replied with a three-pointer for the home side, though, starting a 13-3 run as Ventspils took control for good.
Pawel Leonczyk had 19 points and Mateo Kedzo 18 for Energa, who dropped to 1-4 and saw their hopes of reaching the Top 16 die.
Mareks Jurevičus, player of BK Ventspils:
"The game was tough, especially the end, but we managed to win. Three minutes before the end we were already celebrating the victory and we relaxed. Our opponents used this situation, which made the final seconds of the game more difficult."
Gasper Okorn, coach of Energa Czarni:
"I am satisfied with the performance of the team, because we played without three important players. I liked that players performed like a real team. As I said, I am satisfied with the game, because players showed an attitude. We had problems in defense, basically because we were missing two tall players. Our mistake was that we lost our nerves, including myself. This game was a good experience for us and a good practice."
Pawel Leonczyk, player of Energa Czarni:
"This game was a good practice. We had a chance to win, but we didn't use it so we lost the game."