The hosts broke away in the second quarter, taking a 35-28 lead at the halftime break, which they increased to a game-high 19-point advantage at 55-36, heading into the final break.
However, the Harvey Grant-led Ventspils battled back in the final frame, cutting the gap to six points, as the final buzzer sounded.
Travis Leslie led the winners in scoring with 11 points and also added eight rebounds, Chris Cooper and Julius Jucikas chipped in 10 points apiece for Siauliai.
For Ventspils, Harvey Grant had a game-high 15 points and 10 rebounds, with no other player reaching double digits. Lorenzo Williams chipped in with 9 points and six assists.