West Ham ended a 17-year wait for a Premier League victory at Stamford Bridge as Aaron Cresswell's strike saw them beat Chelsea 1-0 to ease the pressure on manager Manuel Pellegrini.
Cresswell scored the decisive goal three minutes into the second half, curling a low right-footed shot beyond Kepa Arrizabalaga, and the Hammers' victory would have been more secure had Michail Antonio not had a second ruled out by VAR.
- Premier League Table | Fixtures | Top scorers
- How to watch Premier League highlights
- Chelsea 0-1 West Ham: Player ratings
Chelsea registered 19 shots on goal but, with Tammy Abraham out injured, the Blues lacked the clinical edge as they slipped to back-to-back league defeats under Frank Lampard for the first time.
West Ham's first victory away at Chelsea since 2002 ends an alarming eight-game winless run and sees them climb up to 13th in the table.
Chelsea: Kepa (6), James (6), Zouma (5), Tomori (5), Emerson (6), Jorginho (6), Kovacic (6), Mount (5), Pedro (5), Pulisic (6), Giroud (5).
Subs: Kante (6), Willian (5), Hudson-Odoi (5).
West Ham: Martin (8), Fredericks (7), Balbuena (8), Ogbonna (7), Cresswell (8), Noble (7), Rice (7), Fornals (7), Snodgrass (7), Anderson (6), Antonio (7).
Subs: Yarmolenko (6), Haller (5), Masuaku (n/a).
Man of the Match: David Martin.
Hammers haunt Lampard's Chelsea
Winless in their last seven meetings with Chelsea and without a win at Stamford Bridge since 2002, the odds were stacked against West Ham getting a result to ease the pressure on under-fire manager Pellegrini. And those odds would have increased significantly had Chelsea converted one of a host of early chances.
Pedro saw a goal-bound effort blocked inside three minutes and, after Antonio blazed over from Cresswell's cross, Mason Mount drew David Martin into his first save in the Premier League before Christian Pulisic sent a free header over the Hammers' bar.
The woodwork came to West Ham's rescue on 22 minutes to prevent a deflected Reece James cross from handing Chelsea an inadvertent lead, while Kepa bailed the Blues out with a stunning save at point-blank range to stop Antonio heading in from Robert Snodgrass' sumptuous cross.
West Ham grew in confidence and stature from there on, but Chelsea still posed a threat. A triple save from Martin thwarted Mateo Kovacic, Giroud and Pedro in quick succession before Pulisic missed a glorious chance to send the hosts into the break ahead when he fired wide from James' cross in stoppage-time.
And, three minutes into the second half, the hosts were made to pay for their profligacy. Pablo Fornals released Cresswell into space in the area, he checked inside James and produced an assured finish into the bottom right-hand corner.
- Olivier Giroud made his first Premier League start since August and Mason Mount, Pedro, Emerson and Fikayo Tomori returned as Chelsea made five changes from the Champions League draw at Valencia.
- Goalkeeper David Martin made his Premier League debut in goal for West Ham and Michail Antonio, Fabian Balbuena and Pablo Fornals were recalled as the Hammers made four changes from the defeat to Tottenham.
West Ham looked to have doubled their lead on 67 minutes when Antonio bundled Snodgrass' cross in at the back post, but the forward was adjudged to have handled the ball and the effort was chalked off.
That left Chelsea with 20 minutes to throw everything at their London rivals in search of an equaliser, but it did not materialise as a resolute defensive display saw the Hammers home and left goalkeeper Martin with a clean sheet and in tears after his winning debut.
Man of the match - David Martin
A Premier League debut, a clean sheet and a vital win. It really was the perfect afternoon for the 33-year-old goalkeeper. In truth, he didn't have too much to do across the 90 minutes but when called into action he was assured and denied the home side, underlining his position as the safest pair of hands currently at West Ham's disposal.
Martin: Dreams do come true
West Ham goalkeeper David Martin told Sky Sports: "It doesn't feel real. I'm 33 now so I've had a decent career and played a lot of games, but it can't prepare you coming into this.
"I've been struggling to eat for a couple of days and Cress (Aaron Cresswell) was laughing at me before the game because I couldn't finish my food - they had to take my plate away! But I enjoyed it and I'll enjoy it even more now."
What the managers said…
Chelsea boss Frank Lampard: "This is a reality check for everyone; me, the players, the fans. We have all been pretty happy with a lot of things so far but today is a reminder of the harsh realities of the Premier League and the realities of where we are at.
"We have to work and improve because we were not good enough. I haven't had to say that this season for an entire game. We created chances but we didn't push on. West Ham were hungry, and they beat us with determination and by digging in."
West Ham boss Manuel Pellegrini: "The key was not to concede easy goals, it's impossible to win when you do that. We were consistent and concentrated. From the beginning the players were absolutely sure of the way we had to play. We tried to win the game from the first whistle, we played with high intensity, pressed high and created chances and options and every player did their duty."
- West Ham earned their first Premier League away win against Chelsea since September 2002, ending a run of 13 without victory at Stamford Bridge in the competition (D4 L9).
- Chelsea lost their first Premier League London derby at Stamford Bridge since April 2018 (1-3 vs Spurs), having gone seven without defeat before today (W6 D1).
- Chelsea didn't concede a single foul in this game, the first time they've ever done so in an entire Premier League game since we have this data recorded (2003-04).
- Chelsea are yet to gain a single point from a losing position in the Premier League this season, losing all four games in which they've fallen behind this term.
Chelsea host Aston Villa and West Ham travel to Wolves in the Premier League on Wednesday at 7.30pm.
The Premier League action keeps coming - make sure you don't miss a thing by signing up for goal alerts with the Sky Sports Score Centre app.
This season, you also can watch Premier League match highlights for free - without being a Sky Sports subscriber. You'll find highlights from every Premier League game - from 5.15pm on a Saturday or shortly after full-time on other days - in the Score Centre, as well as on the Sky Sports website and app plus the Sky Sports Football YouTube channel.
You can follow dedicated live blogs of every Premier League game on the Sky Sports website and app and track goals as they go in with our vidiprinter page.