George Ford's 77th-minute penalty all but crushed Leinster's Champions Cup ambitions as Bath won 19-16 at The Rec on Saturday.
The England number ten finished with 14 points but it was the final three that mattered most, coming a few minutes after Ireland fly-half Jonathan Sexton had missed a tougher chance to also snatch the lead.
Bath enjoyed set-piece dominance more or less throughout thanks to the punch packed by their scrum but Leinster clung on to the contest and had a chance to win at in the closing stages, only for Sexton's long-range penalty to come up short.
Aided by the travelling support a record crowd packed into The Rec, quite impressive given Bath have been around for 131 years, to witness a repeat of last season's quarter-final that was low on try-scoring chances but high on tension.
Given that Leinster next face a double header against Toulon, it's difficult to now see them making the quarter-finals even if they return to Dublin with a losing bonus point.
Sexton opened the scoring with a penalty after eight minutes, quelling Bath's faster start, as the Ireland fly-half looked to make amends for last week's frustrating defeat.
Despite some impressive early defence from Ben Te'o in particular, Leinster's line appeared full of holes with Bath unable to find the right one at the right time.
The scrum was a problem for Leinster last Sunday against Wasps and a thundering drive from Bath set up Ford to level the scores with a penalty from close range.
The England fly-half then slotted an easy drop goal with little pressure on to double Bath's tally and put them ahead for the first time at 6-3.
Bath captain Stuart Hooper had a try chalked off following an earlier knock-on by Matt Banahan with half an hour gone, the best chance of the match up to that point.
Clear dominance at the scrum however wasn't capitalised on initially by the Bath backs, as they headed down the tunnel only 6-3 ahead.
Ford and Sexton traded penalties at the start of the second half, with the Ireland fly-half's strike taking him over 500 points in Europe, as the contest remained cagey.
Time and again Bath threatened to open Leinster up when putting the ball through the hands, but the final moment of brilliance was still missing.
Sexton struck again from over 40 metres again on the angle to level it up at 9-9.
Bath certaintly couldn't complain about lacking a platform, the scrum still bullying penalties out of Leinster as they gambled by going to the corner rather than opting for the posts on 58 minutes.
Once the maul was stopped it appeared to be a poor decision, however a succession of scrums short of Leinster's line eventually yielded a penalty try for multiple set-piece offences with Bath's pack firmly in control.
When all was starting to look lost for Leinster the replacement flanker Josh van der Flier brought huge energy to the contest as he burst onto a pass and had enough power to ground the ball over the line, Sexton balancing the books again at 16-16.
Even the scrum now was suddenly going Leinster's way – a timely penalty on 72 minutes handing Sexton a crucial attempt from just inside his own half on the edge of his range. The direction was spot on, but it lacked the legs.
The scrum instead produced a Bath penalty to give Ford a chance of his own, again on the angle but far close than Sexton's effort minutes beforehand.
He made no mistake, threading the ball down the middle to make it 19-16.
The possibility of an upset remained on the cards after Bath replacement Alafoti Fa'osiliva was sin-binned for a dangerous tackle but Leinster couldn't find that much-needed chance to win the game, with the three-time European champions' hopes now hanging by a thread.
Tries: Penalty Try
Pens: Ford 2
Drop Goal: Ford
Yellow Card: Fa'osiliva
Tries: Van der Flier
Pens: Sexton 3
Bath: 15 Anthony Watson, 14 Semesa Rokoduguni, 13 Ollie Devoto, 12 Kyle Eastmond, 11 Matt Banahan, 10 George Ford, 9, Chris Cook, 8 Leroy Houston, 7 Francois Louw (c), 6 Matt Garvey, 5 David Attwood, 4 Dom Day, 3 Henry Thomas, 2 Rob Webber, 1 Nick Auterac.
Replacements: 16 Ross Batty, 17 Nathan Catt, 18 Max Lahiff, 19 Stuart Hooper, 20 Alafoti Fa'osiliva, 21 Niko Matawalu, 22 Rhys Priestland, 23 Tom Homer.
Leinster: 15 Isa Nacewa (c), 14 Fergus McFadden, 13 Ben Te'o, 12 Luke Fitzgerald, 11 Dave Kearney, 10 Johnny Sexton, 9 Isaac Boss, 8 Jamie Heaslip, 7 Jordi Murphy, 6 Rhys Ruddock, 5 Hayden Triggs, 4 Devin Toner, 3 Mike Ross, 2 Sean Cronin, 1 Cian Healy.
Replacements: 16 James Tracy, 17 Jack McGrath, 18 Martin Moore, 19 Dominic Ryan, 20 Josh Van Der Flier, 21 Luke McGrath, 22 Ian Madigan, 23 Zane Kirchner.
Referee: Jérôme Garcès (France)
Assistant Referees: Salem Attalah (France), tbc (France)
TMO: Bruno Bessot (France)