Match ends, Chelsea 0, Leicester City 1.
Manager Maurizio Sarri said his Chelsea players reacted with "mental confusion" as they failed to respond to the predatory Jamie Vardy strike that earned Leicester victory at Stamford Bridge.
The Blues lost at home for the first time this season after Vardy's first-time finish in the box from James Maddison's pass in the 51st minute.
Chelsea had been dominant before the break and struck the bar through Eden Hazard following a Harry Maguire error - the best of a host of chances.
But having gone behind, it was Sarri's side who became sloppy as Leicester grew in strength having resisted heavy pressure.
The Foxes would have doubled their lead but for Cesar Azpilicueta's sliding block to deny Vardy after Kepa Arrizabalaga spilled a cross, before the keeper saved well from Marc Albrighton.
Chasing a goal of their own, Chelsea struggled to break their opponents down before missing two key late chances.
In the 89th minute, Antonio Rudiger nodded just wide from a corner, before Marcos Alonso struck the post with only the keeper to beat two minutes later.
"We played well for 55 minutes," Sarri told BBC Sport.
"I think that, of course, in some situations we were unlucky but we could have done something better.
"After the goal the reaction was not the right reaction. We reacted a little shocked and with mental confusion. We had to only keep playing like we did before the goal, there was time to recover."
The Blues had not lost at home since 1 April, against Tottenham, and this defeat leaves them level on points with fifth-placed Arsenal, who beat Burnley 3-1 earlier on Saturday. They trail leaders Liverpool by 11 points.
For Leicester, the win moves them up to ninth in the table and is their first victory from 11 matches in London.
Crisis? What crisis?
Reports before the game spoke not just of pressure on Leicester boss Claude Puel from the club's owners but of an apparent falling-out with his players, too.
There was little sign of the team lacking any motivation or desire on the pitch on Saturday.
Resisting the intensity of Chelsea's attacking intent is by no means a small feat. Puel's men not only managed to achieve that, but also to build on it as they forced themselves slowly but surely into the game.
It was Vardy's goal, scored on his 250th appearance for the club, that sealed victory, but it was a win earned by the whole team through constant running, daring in defence, and precision when it counted.
With Manchester City up next on 26 December, it had been suggested Leicester's following match at home to Cardiff on 29 December might even decide the future of their French manager.
Instead, this impressive victory, a first at Stamford Bridge since 2000, must surely stand as evidence of Puel's worth.
Chelsea however, will now face questions of their own.
Bad misses or bad luck?
This was not a vintage Chelsea performance - not the kind of showing we have come to expect from them at home since Sarri's arrival in the summer.
Looking back on the match, the Italian will identify three big misses that cost his side.
There was Hazard, smashing against the bar in search of his 100th Chelsea goal, there was Rudiger's header wide under pressure but with the goal gaping at a corner, and finally Alonso's 91st-minute miss when clean through.
The Spaniard, played in behind the Leicester defence, side-footed his shot against the post and out, having been given what seemed an age to pick his spot.
Perhaps these were moments of bad luck. Sarri is renowned as being a very superstitious man, but he will not have failed to observe the clear dip in intensity from his side after going behind, which ultimately defined their afternoon.
They seemed shell-shocked after Vardy's powerful strike, and only managed to conjure something of their finer play in the final few dramatic minutes. It turned out to be too little, too late.
Hazard left the field with a wry smile having missed out on the chance of becoming just the 10th Chelsea player to reach that landmark total of a century of goals.
Had he done so, the afternoon might have ended very differently for Puel, the man who gave him his senior debut 11 years ago for French club Lille.
Man of the match - Ricardo Pereira (Leicester)
Chelsea next play at Watford in the Premier League on Wednesday, 26 December, before travelling to Crystal Palace on Sunday, 30 December.
Leicester host Manchester City on Wednesday, before Cardiff travel to King Power Stadium on Saturday, 29 December.
What the managers said
Leicester manager Claude Puel, speaking to BBC Sport:
"It was fantastic hard work from the team together, first to defend and after to try and counter-attack.
"In the second half with the confidence, we performed, scored a fantastic goal, a fantastic reward for the team because we gave our best.
"It is a fantastic feeling. Hopefully we continue this way but the win doesn't change anything. It is important we go away to analyse our game; that we continue to work with the same mentality and same attitude."
Chelsea boss Maurizio Sarri, speaking to BBC Sport:
"As I said at the beginning of the Premier League, it is impossible to catch Liverpool and Manchester City. We have to be careful to stay close to the first four positions."
Vardy a big six menace - the stats
- Chelsea have failed to score in two of their past four home league games, as many as in their previous 21, and lost at home in the league for the first time in 13 games
- Vardy has scored 13 goals in his past 15 Premier League appearances against the 'big six' sides
- Chelsea conceded a home Premier League goal for the first time in four games, ending a run of 357 minutes without an opposition goal at Stamford Bridge
- Leicester's James Maddison has created 41 goalscoring chances in the Premier League this season, more than any other English player
- 30David Luiz
- 5JorginhoSubstituted forFàbregasat 75'minutes
- 17KovacicSubstituted forLoftus-Cheekat 61'minutes
- 10E Hazard
- 22WillianSubstituted forGiroudat 61'minutes
- 14Ricardo Pereira
- 24MendyBooked at 81mins
- 25NdidiBooked at 31mins
- 9VardySubstituted forIheanachoat 84'minutes
- 10MaddisonSubstituted forGrayat 82'minutes
- Lee Probert
- Shots on Target
Second Half ends, Chelsea 0, Leicester City 1.
Marcos Alonso (Chelsea) hits the left post with a left footed shot from the centre of the box. Assisted by N'Golo Kanté.
Offside, Leicester City. Demarai Gray tries a through ball, but Kelechi Iheanacho is caught offside.
Attempt missed. Antonio Rüdiger (Chelsea) header from very close range misses to the right. Assisted by Pedro with a cross following a corner.
Corner, Chelsea. Conceded by Ricardo Pereira.
Attempt missed. Kelechi Iheanacho (Leicester City) left footed shot from outside the box misses to the right. Assisted by Hamza Choudhury.
Substitution, Leicester City. Kelechi Iheanacho replaces Jamie Vardy.
Hand ball by Ruben Loftus-Cheek (Chelsea).
Substitution, Leicester City. Demarai Gray replaces James Maddison.
Ruben Loftus-Cheek (Chelsea) wins a free kick in the defensive half.
Foul by Wilfred Ndidi (Leicester City).
Attempt saved. Marc Albrighton (Leicester City) right footed shot from outside the box is saved in the bottom left corner.
Nampalys Mendy (Leicester City) is shown the yellow card for a bad foul.
Foul by Cesc Fàbregas (Chelsea).
James Maddison (Leicester City) wins a free kick in the attacking half.
Foul by César Azpilicueta (Chelsea).
Ben Chilwell (Leicester City) wins a free kick on the left wing.
Attempt blocked. Eden Hazard (Chelsea) right footed shot from the centre of the box is blocked.
Substitution, Chelsea. Cesc Fàbregas replaces Jorginho.