Thomas drove the 40 lap race solo after his father and co-driver David withdrew with illness following the morning’s practice session at Sandown.
Taking the lead from Kostera with just two laps to go, Thomas held his fellow Mitsubishi Lancer Evo X pilot at bay to bring home the victory.
“Stuart made a mistake out of Turn 4 and that’s where I was able to get passed,” said Thomas afterward.
“Then I missed a gear on the final lap and I thought he would get passed, but he didn’t. It’s a sensational result to win it with two laps to go like that.”