Leinster needed a late penalty to edge Harlequins 14-13 in Saturday's European Champions Cup clash at the Aviva Stadium.
A losing bonus point for Harlequins means that the sides are now level at the top of Pool 2 with two rounds to play.
Leinster led 11-0 at the interval thanks to a try on the half-hour mark from scrum-half Isaac Boss and two penalties from the boot of Ian Madigan.
But Quins launched an comeback in the second half, based on their dominant scrum, and full-back Mike Brown's try saw the visitors take a two-point lead into the final ten minutes.
But Madigan landed a 71st-minute penalty to put the three-times champions back in front in a tense finale thick with controversy.
A chilly night in Dublin offered perfect conditions for both sides to throw the ball around, but the early exhanges in another bruising encounter between these two evenly matched sides saw little in expansive game play.
Quin's dominance in the scrum offered them a quality platform but Leinster's doggedness saw them steal the spoils.
The first chance for points came after an overly-eager Jamie Heaslip thought the ball was out and came around the side of a ruck. Referee Romain Poite blew for an infringement and gave Quins fly-half Tim Swiel his first shot at goal. An awkward-looking kick went wide.
Loose kicking from both sides early on underlined the nervousness. Danny Care tested the home side's defence around the fringes.
Leinster had plenty of possession without any real penetration. Sean Cronin offered the best option to get over the gain line.
Jack Clifford failed to clear the tackle areas correctly but Madigan struck the upright with the resulting penalty.
Moments later Clifford gave away another penalty, which allowed Leinster setup a maul from a five meter lineout.
Dominic Ryan drove hard for the line but was held up short. Isaac Boss was then adjudged to have played the ball on the ground inches from the line and gave away a penalty.
Quins had to make a number of tackles with Leinster enjoying a large share of possession.
George Robson was caught in an offside position in his own half, which gave Madigan a chance to redeem himself from his earlier miss. He made no mistake with his second effort.
Three points was the least Leinster deserved from the amount of possession they enjoyed.
A loose clearing kick from Care in his own 22 gave Gopperth open field to run into. He linked up with Zane Kirchner on the far touchline. The South African attacked the 'Quins 22 and a chip kick over the top forced Lowe to carry it into touch.
The pressure was on and the crowd could smell blood.
Moments later from a close-range scrum Heaslip snuck down the blind side and fed Boss to slide over in the corner. Madigan missed a difficult conversion from the touchline.
Leinster then naively showed ill-discipline from the restart and gave a penalty away right in front of the post.
Swiel had a chance to silence the buoyant crowd, but missed with a poor kick.
Just before the break Poite warned Quins skipper Joe Marler about repeated ill-discipline at the ruck after Quins gave away another penalty.
Madigan stepped up to sink the penalty and send the teams into the break with Leinster leading 11-0.
Moments into the first half Marler cut the Leinster defence apart with a great angle. He trundled up into the Leinster 22. A neat chip kick over the top from Care allowed Mike Brown to run behind the Leinster line, pluck the ball from the air and step Rob Kearney to touch down under the posts.
Controversy then rained as Poite and the TMO came to the decision that Marler had knocked on the ball in the build up after the tackle by Kearney. All evidence seemed to point to interference by the full-back.
Swiel did however convert a penalty after Quins exerted dominance in the scrum deep in the Leinster half. The English club returned to their own half with three points but it could easily have been seven.
The visitors began to rack up the penalties at scrum time with two in quick succession.
A resulting penalty allowed them to set up a driving lineout close to the Leinster line. Quick recycling from Hopper gave Mike Brown an easy run in for the try. Swiel added the extras
The atmosphere turned nervy in the closing 15 minutes. The home fans worrying that the momentum had swung in favor of the visiting side.
An early tackle by Devon Toner in his own 22 gave Swiel another shot at the posts. He dually accepted to move his side into the lead for the first time.
Leinster needed to be playing in the right areas but couldn't seem to find the necessary execution from the boot.
Substitute Eoin Reddan had a point to prove after losing his starting place. He sniped twice from deep and marched Leinster 60 meters up field. Dave Ward then gave away a penalty just outside his own 22. Madigan slotted the kick to restore their lead.
Handbags late on delayed precedings by over ten minutes. The end result was a yellow card for Charlie Matthews for a hand in Dominic Ryan's face.
Leinster failed to put the ball up the jumper but Quins couldn't muster one last shunt in the scrum for a match winning shot at goal. A last minute knock-on saw the home fans breath a huge sigh of relief.
The scorers:
For Leinster:
Try: Boss
Pens: Madigan 3
For Harlequins:
Try: Brown
Con: Swiel
Pens: Swiel 2
Yellow card: Matthews
Leinster: 15 Rob Kearney, 14 Zane Kirchner, 13 Luke Fitzgerald, 12 Ian Madigan, 11 Darragh Fanning, 10 Jimmy Gopperth, 9 Isaac Boss, 8 Jamie Heaslip (c), 7 Dominic Ryan, 6 Rhys Ruddock, 5 Kane Douglas, 4 Devin Toner, 3 Mike Ross, 2 Sean Cronin, 1 Jack McGrath
Replacements: 16 Bryan Byrne, 17 Michael Bent, 18 Tadhg Furlong, 19 Mike McCarthy, 20 Jack Conan, 21 Eoin Reddan, 22 Noel Reid, 23 Mick McGrath
Harlequins: 15 Mike Brown, 14 Marland Yarde, 13 Matt Hopper, 12 George Lowe, 11 Aseli Tikoirotuma, 10 Tim Swiel, 9 Danny Care, 8 Nick Easter, 7 Jack Clifford, 6 Luke Wallace, 5 George Robson, 4 Charlie Matthews, 3 Will Collier, 2 Dave Ward, 1 Joe Marler (c)
Replacements: 16 Joe Gray, 17 Mark Lambert, 18 Kyle Sinckler, 19 Sam Twomey, 20 Joe Trayfoot, 21 Karl Dickson, 22 Tom Casson, 23 Ross Chisholm
Venue: Aviva Stadium, Dublin
Referee: Romain Poite (France)
Assistant referees: Christophe Berdos (France), Stéphane Boyer (France)
Television match official: Eric Gonthier (France)
By Hugh Foyle
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