For a long time, it was as if West Ham had succeeded.
Bayer Leverkusen, unbeaten and fluid, are unable to overcome a relentless defense and some exploits from Lukasz Fabianski.
But then, with seven minutes to go, the defenses were still outplayed.
Before Jonas Hofmann scored a disjointed first game in the 83rd minute, West Ham had faced and survived 24 of the German champions.
Another replacement, Victor Boniface, returned home to give the Hammers an omnipotent task next week in London.
It was a near-perfect performance from an injured team, who narrowly missed a breakaway goal they managed early on.
But the plan worked, until it collapsed too late.
The task is already difficult, but it will be even more so next week with talisman Lucas Paqueta now suspended after a stupid first-half booking meaning he will be suspended for the second leg.
David Moyes is without a quartet of veteran players, with Jarrod Bowen, Alphonse Areola and Kalvin Phillips injured, while Edson Alvarez is suspended.
With an already limited squad and facing a team that could win the club’s first Bundesliga title on Sunday, the Scot will only play one way.
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Two tweaks from the weekend’s win over Wolves saw Moyes move to the full-backs tasked with thwarting the daylight of Xabi Alonso’s fluid story-makers.
Despite all of Leverkusen’s early possessions, they created few transparent chances.
It was West Ham who deserved to have opened the scoring and given everything to defend.
Michail Antonio, who has made a huge difference in his attacking risk since returning from injury, unleashed superbly down the left.
Mohammed Kudus was alone in the centre of the box and saw through Antonio, but fired his first shot straight at goalkeeper Matej Kovar when he had time to decide his place.
With Bowen out, Moyes Kudus will step in at times like this.
The Hammers boss had called for composure on a night when European experience was needed, but his team were worried and ran for the ball.
Paqueta, who steals the exhibition with his feet, took the center of the level after just 21 minutes and sparked a big fight with a frustrated and delayed challenge to Amine Adli.
A hard shot from Alejandro Grimaldo, saved by a Fabianski in flight, had already turned up the volume before the Brazilian’s caution infuriated the home fans.
He was lucky to escape a red card moments later after a silly hit to the feet of Czech striker Patrick Schick.
The yellow card didn’t put the playmaker on the line for the rest of the night, but it also means he will miss the second leg in London next week, a massive loss for West Ham with Bowen already in doubt.
While those in front were worrying and fighting, Fabianski was busy. The Pole deflected another shot, this time from Schick, wide of the far post before preventing Edmond Tapsoba.
On the other side, Antonio is ploughing a lonely furrow, even if it’s a role he’s used to.
The Jamaican foreigner made his way from the halfway line to the penalty area in front of 3 Leverkusen defenders, to see his cross cut out before reaching Paquetá who was waiting.
Alonso’s men controlled 18 shots in the first half, but West Ham held on and even controlled to create the group.
All they had to do was start from the back with a very limited bench, made up of two goalkeepers from the academy and another 4 youngsters from the field.
The long-suffering visitors continued to absorb the tension after the restart, dealing with the constant tension and Fabianski’s goal.
Tomas Soucek and James Ward-Prowse were temperamental on the ball but impressive in protecting the back five, restricting Leverkusen star Florian Wirtz to a minor role.
Each attack or cross revealed a white blouse in its wake, Vladimir Coufal nearly falling unconscious as he firmly received a cross from Grimaldo to his face.
When the locals broke through, Fabianski prevented everything from happening.
He again contained Schick, firing a hard header over the bar in the 70th minute.
Alonso made substitution after substitution, a luxury that Moyes did have.
They were still holding on, and Fabianski, who turns 39 next week, turned off Hofmann in a one-on-one.
But it may just not do anything for the winner.
Boniface received his shot from a corner blocked by Zouma on the ground, but the ball came to Hofmann and he swept the goal to nullify the master plan.
Then the blockade broke out. Soucek stretched to block the line before Boniface crashed in stoppage time, heading in Hofmann’s cross from a short corner.
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