In the top of the eighth, Valley de Oro took a commanding three-run lead, but Calexico bounced back.
Jose Montel, Jeremy Roldan, and Chris Garcia all scored runs in the bottom of the inning to force a ninth.
In the ninth, Trevor Robinson got on with a single and Roldan and Chris Garcia both walked to load the bases.
Marcus Niebla then hit a line drive over second base that drove in Robinson with the winning run.
"We really clicked in this game," Acuña said. ‘They never gave up and they did their best."
Calexico advances to the championship round to face South Bay today.
Acuña said he is proud his team is representing the Imperial Valley in the final round.
"It's like a dream for them," Acuña said. "They really followed through as a team."
"I was really, really happy for them," Acuña said. "They really deserved this win."
Acuña said he is confident about today's matchup.
"If we hit like we usually hit, we're going to come through and win this game," Acuña said.
He added he intends to start his ace, left-handed Esau Madrigal, at pitcher.
Calexico will face South Bay today at 5 in Imperial Beach and needs to defeat South Bay twice to take the tournament because of the double-elimination format.
If South Bay wins either game today, it will take the championship.