Stephen McDonnell's first goal of the season rescued Dundalk as the ten-man Lilywhites came from behind to secure a 2-2 draw, and deny Waterford United victory, in the first leg of the Airtricity League promotion/relegation play-off at Oriel Park.
The home side went in front through a Lorcan Shannon effort midway through the first half, but they were reduced to ten men when Stephen Maher saw red in the 33rd minute.
The Blues deservedly pulled level through a Paul Phelan header (50) before substitute Ben Ryan made an instant impact to give Waterford the lead (60).
Dundalk, though, hit back and McDonnell's 25-yard drive (75) means the tie is on a knife edge going into Friday night's second leg at the RSC.
Buoyed by a run of 11 straight wins on the road, Waterford certainly wouldn't have feared their trip to the Louth venue and they looked bright early on.
Dundalk's first opportunity came in the 20th minute. Shannon's corner from the left was met by captain Chris Shields, but his header flew just off target.
That seemed to settle Darius Kierans' men and they drew first blood seven minutes later. Maher's ball into space set Chris Reilly away on the left.
His first touch took him away from goal, but his second was a beauty, cutting the ball back perfectly for the onrushing Shannon to fire home with his left foot.
Waterford almost replied straight from the kick-off. Sean Maguire cut through the Dundalk back four to set up Phelan to fire an effort goal bound, which Peter Cherrie did brilliantly to tip over, a deflection on the ball making the save even better.
Blues boss Paul O'Brien will have been delighted with the response of his charges to going a goal behind, and they continued to pepper the Dundalk goal, with Cherrie having to be at his best again to deny Maguire and Peter White in quick succession.
Dundalk were on the back foot at this stage, and their hopes of steadying the ship were left in disarray after 33 minutes, when Maher received a straight red card from referee Paul McLaughlin after lashing out needlessly at Peter Higgins on the right touchline.
That gave the visitors more incentive to push forward and Maguire, whose movement caused problems throughout for Paul Whelan and Shields, tested Cherrie again in the 38th minute, but Dundalk held on to take a one-goal advantage in at the break.
The lead lasted for just five minutes of the second-half as Waterford deservedly drew level. Slack marking on the Dundalk right allowed Paul Carey to get his cross away and Phelan was in the right spot at the back post to head the ball home.
The tie was turned on its head ten minutes later. Left back Dylan Mernagh cut back on his right foot to deliver a telling cross over the head of Shields. And Ryan, just seconds after coming on as a substitute, guided a fantastic header across Cherrie to put United in the driving seat.
The ten men refused to buckle though and half-time substitute McDonnell restored parity in the 75th minute with a 25-yard drive that flew past Packie Holden.
Waterford went close to taking the lead again in the 80th minute, but Cherrie again stood firm to deny Gary Dempsey.
Maguire almost won it at the death for the Blues, but his effort went inches wide.
DUNDALK: Peter Cherrie; Mark O'Brien, Chris Shields, Liam Burns (Paul Whelan 13), Eoghan Osborne; Lorcan Shannon, Stephen Maher, John Mountney, Chris Reilly (Stephen McDonnell HT); Marc Griffin, Michael Rafter (Nathan Murphy 85).
WATERFORD UNITED: Packie Holden; Paul Carey, David Breen, Craig Burns, Dylan Mernagh; Peter Higgins (Ben Ryan 60), Gary Dempsey, Peter White, Paul Phelan; Peter Keegan (Darragh Walsh HT), Sean Maguire.
REFEREE: Paul McLaughlin (Monaghan).