West Ham 2 Leicester 3
Leicester made it four wins from four under Craig Shakespeare and picked up their first away victory of the season with a 3-2 success at West Ham.
Riyad Mahrez, Robert Huth and Jamie Vardy - three of the main protagonists of last season's title triumph - stunned the Hammers with first-half goals.
The Foxes showed no signs of a European hangover following their epic win over Sevilla, and although they may have a quarter-final meeting with Atletico Madrid on their minds Shakespeare knows they still need to get their domestic affairs in order.
Fortunately they faced a West Ham side playing as though they just want their 'historic' first season at their new stadium to end as quickly as possible.
Manuel Lanzini briefly halved the deficit with a free-kick but they were booed off at half-time, and although Andre Ayew pulled another back after the break Leicester repelled a late rally to claim all three points.
Rejuvenated under Shakespeare following Claudio Ranieri's sacking last month, the first half was once again a throwback to last season's flying Foxes.
West Ham simply could not cope with the pace and before they knew it the hosts were 2-0 down.
In the fifth minute Mahrez swung in a cross towards Vardy and Shinji Okazaki, but both forwards missed it - as did West Ham's defence - as the ball flew into the corner of the net.
Two minutes later, from a Leicester free-kick, Marc Albrighton clipped the ball into the box with the outside of his boot and the unmarked Huth nodded in the second.
West Ham were dealt another blow after a quarter of an hour when captain Winston Reid went down clutching his leg and had to be replaced.
But they grabbed a lifeline in the 20th minute after Michail Antonio was fouled by Danny Drinkwater and Lanzini curled in a superb 20-yard free-kick.
Ayew and Michail Antonio fired wide and Andy Carroll had a goal ruled out, correctly, for offside as West Ham sensed they could haul themselves back into the game.
However, Vardy restored Leicester's two-goal lead before half-time when he pounced on the loose ball after West Ham failed to clear Albrighton's corner.
Vardy could have added a fourth after the break when he raced on to Mahrez's through-ball and lobbed Darren Randolph, only to clear the crossbar as well.
Instead West Ham pulled another back after Kasper Schmeichel got a hand to a Lanzini free-kick which was not on target, and instantly regretted it as from the corner Carroll's header was turned in by Ayew.
The suddenly re-energised Hammers poured forward but Schmeichel clawed out Carroll's header, Ayew skied a glorious chance and Cheikhou Kouyate's shot was blocked on the line.
And in stoppage time Schmeichel made a stunning point-blank save to deny Carroll as Leicester held out to take another big step away from relegation trouble, and continue to prove there is life in the champions yet.