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Targeting a record-equalling fifth European title, the Irish province ran in three tries inside the opening 12 minutes to leave the holder reeling in one of the great halves of European rugby.
Dan Sheehan crossed twice and Jimmy O’Brien also touched down in a spellbinding opening quarter, but when the second half arrived Leinster appeared to be running on empty.
The second wind arrived too late, coming in response to Georges Henri Colombe’s match-winning 72nd-minute try, and it ended when Michael Ala’alatoa was sent off for a dangerous clear out.
The rematch of last year’s showpiece produced the fastest try scored in a final when Sheehan went charging over after 40 seconds and O’Brien was the next to score.
Both scores bore the hallmarks of meticulous preparation, with Sheehan profiting from the homework done on La Rochelle’s line-out and O’Brien diving over at the end of well-drilled play.
Leinster’s ruck speed was devastating and there were other wins across the field, with James Lowe executing a 50-22 and a turnover on the ground frustrating the Top 14’s second-placed side.
The problems mounted as Tawera Kerr Barlow was shown a yellow card for offside and soon after a defensive lapse allowed an unmarked Sheehan over for his second try.
Trailing 17-0, La Rochelle hit back when Jonathan Danty used his power to bump off Garry Ringrose and touch down after pressure mounted at a five-metre scrum.
Frenzied defence enabled Leinster to secure a penalty that Ross Byrne sent between the uprights – his second in seven minutes – but the heroics came at the cost of losing captain James Ryan to a failed HIA.
La Rochelle was now a different proposition as it started carrying with menace and offloading to support runners, and having stretched the blue wall to breaking point shortly before the interval, it struck to send Ulupano Seuteni over.
Seuteni opened the second-half with an electric break and when Antoine Hastoy kicked a penalty Leinster’s lead was down to six points.
The fireworks that had gone before were replaced by an exchange of penalties and the sight of La Rochelle camping in the Irish province’s 22, grinding away in search of the next score.
Leinster’s defensive resolve was admirable but it was only barely hanging on, its attack no longer functioning, and at times it rode their luck with referee Jaco Peyper.
The yellow tide finally prevailed in the 72nd minute through hard yards from Colombe and the score was compounded by a yellow card for Ronan Kelleher.
Leinster poured forward in the closing stages but their hopes ended when Ala’alatoa received his marching orders, forcing his side to mourn a third successive loss in finals.