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.
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.
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.
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.
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.
Stats: Hammers break Bridge duck
- 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.
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.
Chelsea host Aston Villa and West Ham travel to Wolves in the Premier League on Wednesday at 7:30 p.m. GMT.