Wes Morgan scored to salvage a replay for Leicester
The striker was left seriously embarrassed after his horrendous own-goal put Leicester ahead at a raucous Pride Park.
Yet there is a certain romance when Bent plays on this stage and his predatory header drew the Rams level midway through the first half.
The 32-year-old’s equaliser was his 17th goal in 25 FA Cup outings for Derby, Fulham, Aston Villa, Sunderland, Charlton and Ipswich.
Bent’s latest strike was followed by Craig Bryson’s sweetly-struck effort five minutes before the break and put Steve McClaren’s men on course for a memorable victory.
Leicester, so insipid on their travels this season, kept plugging away and were rewarded when skipper Wes Morgan headed home a corner in the 86th minute.
Derby looked to have battled their way into the fifth round
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McClaren, who lost Will Hughes to a hamstring injury in the second half, said: “I’m disappointed we didn’t win, but what an atmosphere and what a cracking cup tie.
“We did everything we could, but we came across a Leicester team who were at it tonight – just like the Leicester of last season. But our character was fantastic. Darren recovered well, we dug in in the second half and we fought to the end.
“Neither of us wanted a replay but I’m so proud because we showed we have great character in the team.”
At times this season it has been easy to forget that Leicester are the reigning champions. Backed by a good old-fashioned away following of 5,500 supporters last night, though, the visitors led inside the eighth minute.
Marc Albrighton’s right-wing corner was inadvertently headed goalwards by Derby defender Chris Baird.
Bent should have had a simple clearance but the striker slashed at it and could only watch in horror as the ball sliced off his foot and into the net.
Leicester's hopes of a trophy remain alive
The Rams steadied themselves and were level in the 21st minute. After Leicester only half-cleared a corner, Hughes clipped a teasing cross back into the danger zone and Bent cleverly glanced a header past Kasper Schmeichel.
Derby forged ahead in the 40th minute when Bryson collected possession 25 yards out, drove at the heart of Leicester’s defence and swept a delightful low left-foot shot into the bottom corner.
Leicester substitute Demarai Gray hit a post before Riyad Mahrez came off the bench following his return from the African Cup of Nations with Algeria.
The Foxes kept probing and Morgan was on hand to nod home a left-wing corner with four minutes remaining.
Leicester boss Claudio Ranieri said: “I’m satisfied because it was a tough match. In the second half, we created a lot of chances and got the equaliser we deserved. It was important for our confidence.”