A goal from Matt Green gave Birmingham City a 1-0 lead going into half time at the King Power Stadium.
Leicester City got off to a flyer and it took a strong piece of goalkeeping from Darren Randolph to halt a one-on-one from Jamie Vardy.
Yet it was Birmingham who took the lead with their first real opportunity of the game when Shane Ferguson's pinpoint cross was turned in by Green, giving him his first ever goal in league football having previously scored 99 in non-league fixtures.
The next real chance wouldn't come until the 30th-minute mark, when Leicester's Danny Drinkwater found space on the left side of the Birmingham box, but the ex-Manchester United midfielder smashed his effort over.
Leicester were in with another chance soon after when Vardy beat Hayden Mullins, but Randolph got down to make a fine low save to keep out the striker's stinging effort.
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