With so much at stake neither side elected to throw caution to the wind early on with chances at a premium at both ends.
It was Birmingham that took a 16th minute lead though when Gary McSheffrey swung the visitors' first corner of the game to the far post where Radhi Jaidi rose virtually unchallenged to score with a firm header.
And they took just three minutes to seemingly make the game safe when Andrew Cole shook off Nils-Eric Johansson's shirt-pulling challenge to square the ball to the unmarked Sebastian Larsson who found the net with a side-footed shot from seven yards.
Leicester had to wait until the 26th minute for their first attempt on goal when Matt Fryatt curled a 15-yard shot narrowly wide.
And apart from a series of free-kicks aimed too close to the keeper, Leicester failed to conjure up another threat until James Wesolowski failed to hit the target from distance in the final minute of the first half.
Conrad Logan kept his side's faint hopes alive in the 51st minute when he was quickly off his line to foil Cole before Alan Maybury failed to trouble Colin Doyle with a hopeful 30-yarder.
Then Nicklas Bendtner was denied by Richard Stearman's last-ditch tackle as the West Midlanders continued to probe for a third goal to kill the game.
A mazy run by Joe Mattock lifted the home crowd before Doyle pulled off a stunning save to block Stearman's close-range header from Shaun Newton's corner.
With 15 minutes remaining the Foxes threw on Geoff Horsfield as they continued to rally with Doyle called into action to deny first Iain Hume and then Johansson.
Horsfield's physical presence began to unsettle the visitors with the former St Andrew's favourite winning an 82nd minute free-kick which Newton smacked home from 20 yards to set up a pulsating finale.
But Birmingham held on with Doyle twice denying Hume close to the end.