Thomson (27-5) won Game 1, 6-2, while Columbus (29-5) won Game 2, 21-7.
The winner of today's game, which starts at 2 p.m., will advance to the finals against either Oconee County or Ringold this weekend.
"I feel very confident that we can win this series," Thomson first-year coach Jason Osborn said. "We showed them that we can play well by winning Game 1. Unfortunately, they showed us they can win, too. We just didn't pitch very well in Game 2."
It marks the first time in 42 years that the Bulldogs have gotten this far in the state playoffs. The last time, in 1968, Thomson lost a 1-0 game that lasted 16 innings against Americus.
Columbus coach Bobby Howard, who has led the Blue Devils since 1984 and whose teams have won nine state titles, said his team was "fortunate" to have won Game 2.
"Thomson has a very good baseball program, and we were fortunate to have been able to have beaten them," he said. "We hit the ball pretty good in Game 2 and scored a lot of runs. Our kids responded when they had to."
The Blue Devils tagged three different Thomson pitchers for 13 hits in Game 2. Columbus trailed 3-2 before batting around in the top of the third, scoring 10 runs on seven hits. The big inning was capped by center fielder Kyle Carter's grand slam deep over the right-field wall at Randy Jordan Field.
In the fifth, Charles Sheffield launched a three-run homer to make it 21-5.
Thomson returned in the bottom of the fifth and scored two more runs -- solo homers by junior shortstop Nick Rau and senior third baseman Cody Ewest. It was Rau's 13th homer of the season and Ewest's 12th.
Junior left fielder Will Tankersley socked his 12th homer of the season for the Bulldogs -- a two-run shot over the left-field wall in the bottom of the third. Thomson has hit 55 homers this season.
In Game 1, Thomson pitcher Abe Cartledge went the distance, recording his 12th victory against only two losses.
"We've got a lot of heart and soul on this team," the junior said. "It's like a family."
Cartledge yielded just six hits to the Blue Devils.
After Columbus went up 2-1 after three innings, Thomson rallied for four runs on three hits in the top of the fourth, taking a 5-2 lead they never relinquished. The biggest hit was a two-run double by first baseman Andy Grimaud, who collected a pair of hits in Game 1.
Thomson added an insurance run in the seventh, as Rau drew a walk and later scored on a RBI sacrifice fly to center field by Tankersley.