Newly-crowned Bundesliga champions Bayer Leverkusen reached the Europa League semi-finals but almost lost their season-long unbeaten record as they struck late to draw 1-1 away at West Ham United for a 3-1 aggregate victory on Thursday.

Xavi Alonso's side trailed to Michail Antonio's early header and were far below their usual high standards in the first half as the dominant hosts carved out several more chances.

Leverkusen, who clinched their first German title at the weekend, improved dramatically after the break and squandered numerous opportunities before substitute Jeremie Frimpong's shot deflected past Lukasz Fabianski in the 89th minute.

The final whistle sparked loud celebrations from the travelling Leverkusen fans who can now look forward to a semi-final against AS Roma who they lost to at that stage last season. Leverkusen are now unbeaten in 44 matches in all competitions this season and remain on course for a treble.

They came dangerously close to surrendering their proud record, however, and West Ham will rue not being able to take the chances they created after Antonio's goal.

"The intensity and stability with the ball was not good enough in the first half and we were soft in the duels," Alonso told reporters. "We needed to show character and I think the second half we were better and had more control."

Leverkusen looked like they were suffering a post-title hangover as West Ham knocked them out of their stride.

They were sloppy throughout in possession and were punished after 13 minutes when Jarrod Bowen fizzed a superb delivery into the area and Antonio arrived just before keeper Matej Kovar to glance his header into the net.

Another great London night looked on the cards as West Ham's crowd raised the volume and Kovar was almost caught out by a deflected shot by Mohammed Kudus.

The keeper then made a superb reflex save to keep out Bowen's volley that had looked like being a certain goal -- West Ham manager David Moyes falling to the turf in disbelief.

Antonio went close with another header as West Ham made a mockery of Leverkusen's form.

"We could have been two or three up at one point," Moyes told reporters. "I couldn't fault the players for their effort and if we were going to go out of Europe I wanted to go out playing like that."

Whatever Alonso said at halftime clearly worked as the Germans clicked into gear with their slick passing game taking the sting out of the home side.

Frimpong blazed a glorious chance over the crossbar late on after going through all alone and it looked like costing Leverkusen their first defeat for 11 months.

But he redeemed himself when his left-footed effort took a big deflection off Aaron Cresswell.

With their European adventure over, last season's Europa Conference winners West Ham will now try to start another one by finishing high enough in the Premier League, although their recent poor form has seen them slip to eighth.