The striker, a handful all evening, latched onto pass from the left and as everyone with a Spurs affiliation prayed for a linesman's flag, calmly slotted home past Paul Robinson to complete an amazing turn around.
It looked all over when Paul Stalteri smashed home a superb goal from 18 yards to hand us a 2-0 lead after 42 minutes. We had dominated up to then and looked in complete control.
However - and crucially, as it turned out - Elvis Hammond pounced from close range to hand the home side a lifeline at 2-1 just a minute later and lift Foxes tails before half-time. The rest, as they say, is history.
Grzegorz Rasiak was given the chance to impress at the Walkers Stadium as a replacement for the absent Mido, with the only other change from Wednesday's trip to Manchester City being the introduction of Stephen Kelly for the injured Lee Young-Pyo.
Aaron Lennon, once again, began the game full of bristling intent and sped past his marker before delivering a cross that cleared home keeper Rab Douglas and had to be headed to safety by Patrick McCathy with Rasiak lurking.
De Vries at the focal point of the Leicester attack was doing enough early on to suggest he could prove a concern as the game progressed - a header and shot from distance not being too far off the mark - with him being only an inch or two shorter than messrs Dawson and Gardner.
He had a very good chance to give his side the lead when Kelly misjudged a header back to Paul Robinson and the big Dutchman stole in between the two but thankfully his interception forced him wide and the opportunity was spurned.
A free-kick from roughly the midway point in the Leicester half off to the right led to the opening of the scoring on 19 minutes and a fourth goal of the season for Jermaine Jenas. Michael Carrick flighted in the set piece and Robbie Keane connected with a brilliant header that beat Douglas but hit the post and rolled out. Jenas was very much alive to the situation and was first to respond by stabbing the ball into an empty net.
The boys continued to press but the next big chance came at the wrong end when De Vries out-muscled Gardner on the way through and was readying himself to shoot before Dawson did a spot of mopping up behind him. It then could have been 2-0 when Keane was in the clear but Douglas was out quickly and got enough on the ball, although Keane's second attempt did take a deflection and should have won a corner.
We were paying the price of conceding a goal just before the break as a confidence-injected home side went for the jugular
He was again in the thick of the action moments later when a ran onto a throughball and spotted Douglas off his line which was, of course, the cue for an audacious chip that was just too high. Michael Brown then tested the reactions of Douglas down low to his right.
Five minutes before the break our lead was extended in the most spectacular fashion. Stalteri embarked on a run down the right and was duly played in by Keane. While probably the Leicester defenders - and perhaps our own attackers - were anticipating a cross, the Canadian ran the ball to the top corner of the penalty area before unleashing a searing drive off the inside of the post and high into the net.
Celebrations did not last long though and Leicester gave themselves a lifeline before the whistle when Richard Stearman headed a deep cross back into the six yard box and substitute Elvis Hammond was positioned to poke home and make it 2-1 at half-time.
It would be fair to say Leicester were suitably fired up after the interval and, galvanised by their goal, laid immediate seige to Robinson's goal. Robbo lived up to his billing straight away and pulled off a superb stop to deny Stephen Hughes after De Vries teed him up. A minute or two later the former Hearts man shook off Gardner and pulled the trigger again, only for the ball to narrowly clear our crossbar.
The sting was nearly taken out of them on the counter when Keane found space and set Kelly off on a charge down the right. Unfortunately, with goalkeeper to beat, the full-back arrowed his shot wide of the post.
On 57 minutes Leicester were level. A corner from the left was played into the path of Hughes by De Vries and the midfielder smacked the ball - via a slight deflection off Dawson - past Robinson and into the net. The Foxes were now firing.
The boys got a grip again after a few panics and dampened the fires, to an extent, with Lennon once more offering a lightening quick outlet down the right that was posing problems for the home defences.
Just before the 70 minute mark, Jermain Defoe was sent on in place of Brown in a bid to secure a winner on the night, but it was De Vries who next nearly troubled the scorers with a drive that flashed past a motionless Robinson's left hand upright. Defoe did have a shooting chance after being found by Keane, but elected for a chip rather than power in front of goal.
It was real Cup tie stuff and, as the clock ticked down, either team could really have won, with Leicester seemingly having an energy surge and closing down ever so quickly whenever the ball was at the feet of a man in white.
As the fourth official held up his board showing three minutes, De Vries latched onto a pass from the left, controlled and rolled past Robinson. That was that for the FA Cup for another year.
Mark de Vries struck in injury time as Leicester City came from 2-0 down to knock us out of the FA Cup on Sunday.