Ndidi and Gray score fantastic extra-time goals for Leicester
Foxes will travel to League One side Millwall in the fifth round
Watch clips of goals and key incidents on this page
Both sides named weakened teams (10 changes for Leicester, eight for Derby)
Leicester 2-1 Derby
Kevin Kilbane
Former Everton winger on BBC Radio 5 live
One thing he's done since he's come on is passed the ball forward and followed it up. When he gets into those positions he drives forward. What a strike! What a brilliant strike!
Image source, Reuters
Wilfred Ndidi
Can the Leicester substitute make an impact? Ndidi he can! (Sorry.)
Wilfred Ndidi drives the Foxes forward from the halfway line, the Derby defence back off and back off then...wallop!
The £15m signing from Genk smashes in a 20-yarder off the right-hand post and in.
Leicester 1-1 Derby
Tired legs = an open 30 minutes with lots of chances? Let's hope so. Derby are keeping the ball neatly in the Leicester half, but the home side dispossess them and break forward....
Leicester 1-1 Derby
Leicester go straight on the offensive. Riyad Mahrez and Andy King linking down the left, the Welshman's cross gathered by Rams keeper Jonathan Mitchell.
Leicester 1-1 Derby
We're rock and rolling in this FA Cup encore. Leicester have a couple of new faces on the stage - striker Islam Slimani and midfielder Wilfred Ndidi come on. Game-changers?
#bbcfacup
Mark Adamoulas: The ref has had a mare tonight. How many critical decisions can you get wrong in one game?
James: How does the referee not see that?
Lewis Starks: Another AWFUL officiating performance today... Basketball players would be proud of that touch! Stonewall handball
Sarah Taylor: Video referee anyone?
Leicester 1-1 Derby
Martin O'Neill
Republic of Ireland manager on BBC One
I cannot blame the referee for not spotting Derby keeper Jonathan Mitchell's handball. From Mike Jones' angle it looks like he has just chested it.
Image source, AFP/Getty ImagesLeicester 1-1 Derby
Derby should be down to 10 men - no question. And here's why...
Leicester 1-1 Derby
Alan Shearer
Ex-England captain on BBC One
There was definitely more energy in the second half. For the first time in the game, for the goal, they got four bodies in there. It wad a brilliant ball in from Demarai Gray. Andy King got in and gambled.
Leicester 1-1 Derby
Claudio Ranieri and Steve McClaren are both barking out some pearls of wisdom on the pitch. All 22 players listening carefully to their instructions while swigging some colourful electrolyte-packed drink.
Image source, PALeicester 1-1 Derby
Derby hold firm. Another 30 minutes of fun!
Leicester 1-1 Derby
Leicester throwing the kitchen sink at Derby now. And forks, plates, spatulas...the full lot.
Leicester 1-1 Derby
The corner is headed out by Derby skipper Richard Keogh, as is the second ball back in. Centre-half Marcin Wasilewski takes a swing with his left peg...blocked.
Leicester 1-1 Derby
All Leicester now. They're going for it. And they have a corner...
Leicester 1-1 Derby
Kevin Kilbane
Former Everton winger on BBC Radio 5 live
What was Max Lowe thinking? He was trying to put it behind for a corner and got it horribly wrong.
Leicester 1-1 Derby
Three minutes for someone - anyone - to stop this going into extra-time...
Leicester 1-1 Derby
Lucky boy! Derby left-back Max Lowe gets into a tangle trying to deal with a cross from the left, chesting the ball against his own post.
Image source, ReutersLeicester 1-1 Derby
Chance at the other end! Leicester break down the right, Ahmed Musa meeting a cross with his head. Easy save for Rams keeper Jonathan Mitchell.
Leicester 1-1 Derby
Oooohhh, errrrrr. Nervy times at the back for Leicester, Marcin Wasilewski miskicking a cross into his penalty area. The Foxes survive...
Leicester 1-1 Derby
This tie is smouldering now. Can it catch fire in the final few minutes?