The Baggies ran out 1-0 winners over Tottenham Hotspur on Saturday, Jake Livermore forcing the ball over the line in the 92nd minute of the encounter to keep his side's Premier League survival hopes alive.
Albion now sit two points from safety with one game to go, although they will still go down if Southampton win at Everton in Saturday's late kickoff.
Despite still requiring a sequence of events which Foster describes as "a miracle" to remain in the survival race going into the final week, the goalkeeper is nonetheless feeling much more confident.
"It was just a battle," Foster told BBC Sport about their game against Spurs. "We knew Tottenham's quality and we would spend time under a lot of pressure. The lads really put their hearts on the line and were brilliant. We know we are a threat at set pieces and we managed to nick one. It keeps us going for another week.
"Once you get stuck in a rut, I don't anyone really believed [that West Brom could survive]. All the momentum was drained out of us. Everyone will say fair play [caretaker manager] Darren Moore. He has instilled that work ethic, we had that before but confidence wasn't there.
"With the confidence we have now and the momentum, it is a miracle we have got to the last week, if it does go that far we will go with full confidence we can do it. Six weeks ago I had us resigned to be relegation. I accepted our fate was inevitable. Fair play to Darren Moore and the coaches. The mental side is 50-50 with the fitness."
West Brom wrap up the season with a trip to Crystal Palace on May 13.