Mauricio Pochettino suffered his first defeat as Chelsea head coach as second-half goals from Michail Antonio and Lucas Paqueta earned West Ham a smash-and-grab victory on Sunday afternoon at London Stadium.
New signing James Ward-Prowse delivered assists for Nayef Aguerd - who was later sent off for a second yellow card - and Antonio while the Blues were left to rue Enzo Fernandez's penalty miss after Carney Chukwuemeka had equalised.
Summer signing Moises Caicedo came on as a substitute as Chelsea pushed forward following Aguerd's dismissal but gave away a spot-kick which Lucas Paqueta converted in second-half stoppage time to wrap up the scoring.
An early corner forced Robert Sanchez into some flappy action, doing enough to stop Tomas Soucek's outstretched leg powering the ball into the net, but Chelsea's lack of conviction in defence was made clearer as the hosts took the lead.
Ward-Prowse, making his Hammers debut six days after his signing from Southampton was confirmed, whipped a delicious cross corner to the back post where Aguerd was waiting to tower above Conor Gallagher and nod in a simple opener.
The reason for Ward-Prowse's arrival was made clear by that first assist, while other Irons like Soucek and Antonio enjoyed the better of the physical exchanges against Chelsea's young bucks early on.
However, the Blues started to enjoy more of the ball and Nicolas Jackson should have done much better from Ben Chilwell's pinpoint cross, getting under his header from close range.
The visitors struggled to finish off promising moves, with Raheem Sterling failing to play the desired accurate pass on occasion while Chukwuemeka shook off the attention of Vladimir Coufal by the touchline but could then only spoon a deflected cross of no threat into the box. However, a more direct approach paid dividends as Chelsea drew level before the half-hour mark.
Some twinkling feet sent Jackson into the box before a Chilwell cross was again only half-cleared by West Ham, allowing Chukwuemeka to cut onto his right foot away from Soucek and batter in a strike that flashed beyond Alphonse Areola for 1-1.
Some quick feet from Paqueta helped the Brazilian hit the post but Chelsea's grip on the game intensified after the equaliser. Sterling continuously got the better of Emerson Palmieri out wide to drive at a standoffish Hammers defence.
His endeavour brought about a chance for Pochettino's side to go ahead when the England international was bundled over just inside the box by a lazy Soucek challenge. However, Enzo Fernandez's penalty was tame and central, allowing the smart Areola to guess the right way and keep West Ham level.
David Moyes' side really struggled to reach half-time and were clearing efforts and sixes and sevens by the end as Jackson failed to connect with Sterling's cross after another sharp run. Chilwell arrived to place his shot but it was hacked away.
Goalscorer Chukwuemeka suffered a cruel knee injury just before half-time as Mykhailo Mudryk entered the fray as his replacement.
West Ham took the lead in extreme 'against the run of play' fashion, aided by some spectacularly bumbling defending from Axel Disasi. The summer signing dawdled in possession and gave it straight to Ward-Prowse, whose lobbed pass allowed the previously silent Antonio to race in and finish with confidence in the far corner.
That goal soon prompted the debut of new £115m signing Moises Caicedo with the Blues staring at a first defeat of the season, having drawn against Liverpool in their first outing. However, their comeback hopes were given a shot in the arm when Aguerd foolishly slid in on Jackson, earning a deserved second yellow card to exit the field.
Chelsea's huffing and puffing came to little effect. Their summer spending significantly dwarfs West Ham's but the Blues' star arrival Caicedo made a poor judgement call in the 94th minute, colliding into Emerson to allow Paqueta to convert from the spot.
West Ham player ratings (4-2-3-1)
GK: Alphonse Areola - 8/10 - Confident few claims at corners and a penalty save have done the Frenchman's case as starting Hammers goalie the world of good.
RB: Vladimir Coufal - 7/10 - Struggled up against Chukwuemeka but his hard-as-nails style suited the situation as West Ham held on.
CB: Kurt Zouma - 6/10 - Present enough to repel most of what came his way, though was sometimes left stranded by the central midfield pairing.
CB: Nayef Aguerd - 4/10 - An easy goal early on but did everything right in converting Ward-Prowse's cross. What he was doing sliding in on Jackson while on a yellow is anybody's guess. A mixed afternoon, for sure.
LB: Emerson Palmieri - 6/10 - Progressively struggled to contain Sterling to the point where he tried to provoke his opponent having just been left for dead. Later won the penalty off Caicedo.
CM: Tomas Soucek - 6/10 - Looked threatening early on as West Ham probed but looked so sluggish as Chelsea started to monopolise possession. Badly judged tackle on Sterling gave away a penalty which he was fortunate to see Areola save. Much improved in the second half.
CM: James Ward-Prowse - 8/10 - Trademark set-piece delivery earned him a first West Ham assist seven minutes in. Grabbed another one with a smart ball to Antonio.
RW: Jarrod Bowen - 6/10 - Awesome bit of hold-up play late on was probably his best contribution in an otherwise quiet display.
AM: Lucas Paqueta - 7/10 - Probably didn't need to pick up a booking for booting the ball away in his current predicament. Some nice quick feet saw a poked effort hit the post but seemed in a huff for most of the fixture before he netted from the spot late on.
LW: Said Benrahma - 6/10 - Enjoyed fleeting dribbles on occasion and looked in decent touch before being sacrificed.
ST: Michail Antonio - 7/10 - Just the five touches in the first half. That meant little at London Stadium when he held off Colwill and smashed past Sanchez.
Substitutes
Angelo Ogbonna (XX' for Benrahma) - 6/10 - Brought on switfly after Aguerd's dismissal.
Pablo Fornals (76' for Antonio) - 6/10 - Bought West Ham some time with clever feet late on.
Edson Alvarez (81' for Ward-Prowse) - 6/10 - Helped clog the middle of the pitch while Chelsea probed.
Subs not used: Lukasz Fabianski (GK), Ben Johnson, Aaron Cresswell, Maxwel Cornet, Danny Ings, Divin Mubama.
Manager
David Moyes - 7/10 - Moyes' side soaked up pressure to the best of their ability and capitalised from some wasteful Chelsea finishing by being clinical themselves.
Chelsea player ratings (3-4-2-1)
GK: Robert Sanchez - 5/10 - Few sharp stops, including one at his near post from Fornals.
CB: Axel Disasi - 4/10 - Horribly sloppy for West Ham's second goal, pondering on the ball before Ward-Prowse played in Antonio. Couldn't get a retreating block in.
CB: Thiago Silva - 5/10 - Not his usual commanding self, with West Ham's goals coming as a result of Chelsea's disorganisation at the back.
CB: Levi Colwill - 4/10 - Tried to nick the ball off Antonio when holding him up needed to be the play.
RWB: Malo Gusto - 3/10 - A little bit of a headless chicken out wide, struggling to provide any threatening crosses until late on when Chelsea had a player advantage.
CM: Enzo Fernandez - 4/10 - A big part in Chelsea's domination of possession, constantly whipping the ball out wide to Chelsea's wide players. Poor missed penalty cost his side dearly.
CM: Conor Gallagher - 4/10 - Got caught under a cross when Aguerd scored but was more defensively sound screening the back four than in the past. Definitely playing a role he's not suited to.
LWB: Ben Chilwell - 5/10 - Tried to arrive late into the box and nearly got on the end of some crosses. Good ball in which Jackson should have converted.
RF: Raheem Sterling - 7/10 - Few errant passes towards Jackson when precision early on but then got into his groove, winning a penalty that wasn't converted by Fernandez. Link-up play with Jackson so nearly returned a goal on many occasions.
LF: Carney Chukwuemeka - 7/10 - Scored his first ever Premier League goal in a first-half display that included sparking feet and a belter of a strike to equalise. Had to come off just before the break with an injury.
ST: Nicolas Jackson - 6/10 - Made the right runs early on and should really have buried Chilwell's cross. Began to hold up the ball really well but couldn't make good on that promise.
Substitutes
Mykhailo Mudryk (45' for Chukwuemeka) - 4/10 - Needed to go one-v-one more often. One chance to shoot after a great touch was followed by a comical slip.
Moises Caicedo (61' for Chilwell) - 2/10 - On for his debut and quickly went close with an ambitious effort from distance. Gave away a penalty to compound a poor day for Chelsea.
Noni Madueke (75' for Gallagher) - 5/10
Mason Burstow (83' for Gusto) - N/A
Subs not used: Lucas Bergstrom (GK), Marc Cucurella, Ian Maatsen, Bashir Humphreys, Lesley Ugochukwu.
Manager
Mauricio Pochettino - 3/10 - His side typically dominated possession but lackadaisical defending offered West Ham three cheap goals.
