Bluewater broke a 2-2 tie with under six minutes remaining in the third period of the back and forth game.
"We kept it at it. When Bluewater scored, I think we took the play over and tied it," said Barnes. "During a (Bluewater) timeout, we (coaches) said to the girls, play defence and we'll be happy with the tie, but if you get the opportunity go for it, and Kensley did."
Olivia Bosma and Austin Goebel gave St. Thomas leads of 1-0 and 2-1.
Zoe Cyros, with two goals and Clora Campbell, scored for Bluewater.
St. Thomas advanced to the final, following a round robin record of three wins, two ties and a loss, against Bluewater and West Oxford.
"It was tough with the COVID break. It took a couple weeks, but we got our legs back. We've been playing well at the right time," Barnes said. "Coming together positionally was a main thing for our girls. Fundamentally, keeping our positions in check was key. We've done that."
Game two of the final, is Sunday, at Strathroy. Game three, if necessary, is back at Memorial, Monday, March 28.