Claudio Ranieri side Leicester City placed a first home win for 2017 as 20 years old striker Demarai Gray's excellent solo goal secured an extra - time victory against Derby in their FA Cup fourth round.
Wales midfielder Andy King netted opening goal the hosts, who made 10 changes, along after Gray's brilliant croos was acknowledged back across by English midfielder Marc Albrighton.
Guinea striker Abdoul Camara's free kick affected extra time for championship Derby only for substitute Onyineye Ndidi to add one more goal for the Foxes advantage with a fabulous strike.
English striker Gray netted a deserved victory with a bend finish after a fine run.
Leicester City will play against One Millwal in the last 16 on 18 February.
Leicester City manager Claudio Ranieri said: "Derby played good football and we won, This is what we needed and I wanted.
"We want to do well in all competition. We want to go forward in the FA Cup. The premier league is not so good but we have to stay in the premier league. This fresh air is good for the player."
Derby County manager Steve Mclarin: "There are some very tired players in the dressing room. It was always going to be hard work.
" We had a go and I can't fault the players. We ran out of steam in the end. We missed our opportunity in the first game."