Seibutis' 3s gave Lithuania a 63-56 lead with 4:45 remaining in the game. Lithuania, which trailed 54-50 at end of the third quarter, outscored Tunisia 26-9 in the fourth.
A gritty Tunisia squad had a chance to advance with a win but ran out of gas down the stretch.
Mantas Kalnietis helped put the game away with a 3-pointer and a pair of free throws.
Darius Songaila and Sarunas Jasikevicius each finished with 13 points for Lithuania.
Amine Rzig led Tunisia (0-5) with 17 points.