FOR so long it felt like West Ham had pulled it off.
The unbeaten, free-flowing Bayer Leverkusen were unable to get past dogged defending and some Lukasz Fabianski heroics.
But then, with seven minutes to go, the defences were finally breached.
Before Jonas Hofmann swept home a scrappy 83rd minute opener, West Ham had faced and survived 24 attempts from the German champions-elect.
Another substitute, Victor Boniface, then headed home a second to give the Hammers an almighty task next week in London.
It was a near-perfect performance from an injury hit side, just missing a breakaway goal which they should have grabbed early on.
But the plan was working, until it fell apart so late on.
The task was already hard, but will be made all the more difficult next week with talisman Lucas Paqueta now suspended after a silly first half booking means he will be suspended for the second leg.
David Moyes was without a quartet of senior players with Jarrod Bowen, Alphonse Areola and Kalvin Phillips all injured, while Edson Alvarez was suspended.
With an already limited squad and up against a side who are expected to lift the club’s first ever Bundesliga title on Sunday, the Scot was only ever going to play one way.
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Two changes from the weekend win at Wolves saw Moyes move to a back five tasked with frustrating the living daylights out of Xabi Alonso’s free-flowing history makers.
For all of Leverkusen’s early possession, they created little in terms of clear cut chances.
It was West Ham who should have opened the scoring and given themselves something to really defend.
Michail Antonio, who has made such a difference to their attacking threat since returning from injury, broke superbly down the left.
Mohammed Kudus was alone in the middle of the box and picked out by Antonio, but hit a first time shot straight at goalkeeper Matej Kovar despite having had the time to pick his spot.
With Bowen not here, Moyes needed Kudus to step up in moments like that.
The Hammers boss had called for composed heads on a night where European experience was needed, but his side were anxious and rushed on the ball.
Paqueta, usually stealing the show with his feet, took centre stage after just 21 minutes by sparking a mass brawl with a frustrated and late challenge on Amine Adli.
A powerful strike from Alejandro Grimaldo, stopped by a flying Fabianski, had already turned the volume up before the Brazilian’s booking had the home support furious.
He was fortunate to escape a red card moments later following a silly stamp on the toes of Czech striker Patrick Schick.
Not only did the yellow card put the playmaker on a tightrope for the rest of the evening, but it also means he will miss the return leg in London next week – a huge loss for West Ham with Bowen already a doubt.
While those ahead of him were fretting and fighting, Fabianski was rather busy. The Pole tipped another effort, this time from Schick, wide of the far post before stopping from Edmond Tapsoba.
At the other end, Antonio was ploughing a particularly lonely furrow – though it is a role he is accustomed to.
The Jamaica international battled his way past from the halfway line to the penalty box past three Leverkusen defenders, only to see his cross cut out before it reached the waiting Paqueta.
Alonso’s men racked up 18 shots in the first half, but West Ham held firm and even managed to create the best of the lot.
They just had to do it all over again with a very limited bench, which featured two academy goalkeepers and four more outfield youngsters.
The embattled visitors continued to soak up pressure after the restart, facing down the constant pressure and the din of the drum from behind Fabianski’s goal.
Tomas Soucek and James Ward-Prowse were skittish on the ball but impressive in screening the back five, restricting Leverkusen’s star man Florian Wirtz to a bit-part role.
Each attack or cross found a white shirt in its way, with Vladimir Coufal almost knocked unconscious when taking a Grimaldo cross firmly in the face.
When the hosts did break through, Fabianski was there, stopping everything.
He saved once again from Schick, tipping a powerful header over the bar on 70 minutes.
Alonso was making change after change, a luxury Moyes did not have.
They were still holding on, with Fabianski – 39 next week – denying Hofmann one on one superbly.
But he could do nothing about the winner.
Boniface had his prodded shot from a corner blocked by Zouma on the floor, but the ball broke to Hofmann in space and he swept home to undo the masterplan.
Then the dam burst. Soucek sprawled to block on the line before Boniface crashed home a second in added time, heading home Hofmann’s cross from a short corner.
