Rush said: "Everyone is now looking ahead to the last game against Newcastle now, but psychologically, I rate the trip to St Andrews even higher at the moment.
"Those looking for Liverpool to slip will fancy Birmingham to get something next weekend. I believe if we win against Steve Bruce's side, we'll be virtually there and, depending on Aston Villa's result at Southampton, may only need to draw against Newcastle.
"The trip to Birmingham has to be treated as the biggest game of the season. Fourth place is within our grasp now and we're in the right form to win our last two matches."