Match ends, West Ham United 1, Chelsea 0.
West Ham won for the first time under David Moyes as Marko Arnautovic's first goal for the club proved enough to beat a disjointed Chelsea at London Stadium.
The Hammers scored the only goal after just six minutes when Arnautovic curled a deft strike into the bottom corner after exchanging passes with Manuel Lanzini on the edge of the area.
Chelsea responded well before the break but were repelled by an organised West Ham defence and two solid saves from Adrian, retaining his spot in place of the dropped Joe Hart.
Both sides became increasingly ragged in the second half but West Ham desperately held on, with Chelsea striker Alvaro Morata firing wide when clean through late on.
Despite victory, the hosts remain in the bottom three, though they have drawn level on points with 17th-placed West Brom, who lost 1-0 to fellow strugglers Swansea later on Saturday.
Defeat means champions Chelsea will finish the weekend 14 points behind leaders Manchester City if Pep Guardiola's side beat rivals Manchester United on Sunday.
- 'If you lose four games in 16 you're not in the race' - Chelsea boss Conte rules out title
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- Relive the action from London Stadium
The Moyes revival
Having ultimately failed to hang on in a vastly improved performance against Manchester City last weekend, West Ham took the next step in proving they could be revitalised under Moyes by this time beating superior opposition.
Their new endeavour was epitomised by Arnautovic, the often frustrating Austria international exchanging positions with his fellow forwards to routinely test Chelsea's defence.
When makeshift centre-forward Michail Antonio drifted out right early on, Arnautovic cleverly moved inside, collecting Antonio's pass and playing a quick one-two with Lanzini to leave Andreas Christensen behind, before calmly finishing low past Thibaut Courtois.
He remained a threat even when West Ham dropped deeper and was denied a decent penalty shout when he tried to go past Christensen and the ball hit the Danish defender's outstretched hand.
At the other end, the Hammers showed greater resolve in defence, largely due to Moyes' organisation of the central back three and wing-backs, with Arthur Masuaku particularly impressive down the left.
It could also be a result of Adrian's more measured presence in goal, exuding more calm to his defence perhaps than the erratic Hart, who could not play against his parent club last week and has now lost his place.
"I have two really good goalkeepers - Joe's professionalism when I told him he wasn't playing was excellent, Adrian has done well again and kept the clean sheet so we're happy," said Moyes.
Whatever the main reason, Moyes will surely stick with Adrian going forward and England's number one Hart could find himself short of first-team football before next summer's World Cup.
The next test for West Ham is to replicate these performances against the teams around them and drag themselves away from danger.
Man of the match - Marko Arnautovic (West Ham)
Is Chelsea's title defence over?
Even before the halfway mark in the season, it already looks like a second successive Premier League title is beyond Chelsea, who trail the imperious Manchester City by 11 points with the leaders still to play this weekend.
Antonio Conte's side have now dropped 16 points in as many games, having dropped just 21 in the whole of last season.
As with defeats by Burnley and Crystal Palace, they were poor against less-gifted opposition and the Blues attack was largely innocuous throughout.
Adrian made two fine saves low to his left to keep out N'Golo Kante and David Zappacosta, but both efforts were somewhat speculative and Eden Hazard displayed none of his recent stellar form as he was unable to exploit West Ham's deep line.
It took a momentary lapse in concentration by Masuaku to create Chelsea's best chance, as the wing-back played Morata onside only for the Spain striker to pull his shot wide, before Hazard struck over in the closing stages.
Nothing Chelsea did was particularly bad, but neither was it that inspiring - a microcosm of a uneven season in which they look set to be left fighting for second at best.
'The fans know West Ham are on the way back' - manager reaction
West Ham manager David Moyes, speaking to BBC Sport: "We're having to stop conceding goals - we have a terrible goals against record - but we've also got people here who can score and cause problems and we've done much better again.
"The supporters showed they know West Ham are on their way back and they're showing that the players are putting in the effort.
"I'd love to play the way Chelsea play but we can only do so much at a time. We've beaten the champions so it's a great achievement for us and a great three points."
Chelsea manager Antonio Conte, speaking to BBC Sport: "When you lose a game you must be disappointed. Our start was bad but we played a bit tired. To face one game every two days always with the same players is very difficult. And we have just started this period.
"We created many chances but were not able to score. We have to go and to play game by game, to be focused. Don't forget in this league it is very difficult. There are six top teams and four go to the Champions League. We have to fight and do our best."
Derbies spell danger for Conte - match stats
- Five of Conte's nine Premier League defeats as Chelsea manager have been in London derbies (Crystal Palace twice, Arsenal, Tottenham and West Ham).
- Moyes secured his first Premier League home victory in just under a year since Sunderland beat Watford on 17 December 2016, ending a run of 11 successive league home games without a win (D5 L6).
- Arnautovic became the 140th different player to score a Premier League goal for West Ham - only Tottenham (142) have had a higher number of players score for them in the competition.
- Arnautovic became the 43rd different player to both score for and against West Ham United in the Premier League - a higher tally than any other club in the competition.
- Courtois has conceded 100 goals in the Premier League for Chelsea in 107 appearances in the competition. It took Petr Cech 172 Premier League appearances to concede 100 goals for the Blues.
- This was the 100th Premier League win that Mark Noble has played in for the Hammers - the first player to reach this landmark for the club.
Chelsea travel to Huddersfield on Tuesday, with kick-off at 20:00 GMT, before hosting Southampton on 16 December at 15:00.
West Ham host Arsenal on Wednesday at 20:00 and are away to Stoke next Saturday at 15:00.
- 13AdriánBooked at 58mins
- 2ReidBooked at 67mins
- 3CresswellBooked at 68mins
- 14ObiangBooked at 70mins
- 26MasuakuBooked at 87mins
- 7ArnautovicBooked at 7minsSubstituted forSakhoat 69'minutes
- 30AntonioSubstituted forA Ayewat 79'minutes
- 20A Ayew
- 21ZappacostaSubstituted forWillianat 64'minutes
- 14BakayokoSubstituted forPedroat 45'minutes
- 3AlonsoBooked at 18minsSubstituted forMosesat 55'minutes
- 10E Hazard
- Anthony Taylor
- Shots on Target
Second Half ends, West Ham United 1, Chelsea 0.
Attempt missed. Cesc Fàbregas (Chelsea) right footed shot from outside the box is too high.
Arthur Masuaku (West Ham United) is shown the yellow card.
Attempt missed. Eden Hazard (Chelsea) left footed shot from outside the box is too high. Assisted by Álvaro Morata.
Andreas Christensen (Chelsea) wins a free kick in the defensive half.
Foul by Diafra Sakho (West Ham United).
Attempt missed. Álvaro Morata (Chelsea) right footed shot from the centre of the box is close, but misses to the left.
Offside, West Ham United. Winston Reid tries a through ball, but Diafra Sakho is caught offside.
Foul by Willian (Chelsea).
Mark Noble (West Ham United) wins a free kick in the defensive half.
Substitution, West Ham United. André Ayew replaces Michail Antonio because of an injury.
Delay over. They are ready to continue.
Delay in match Michail Antonio (West Ham United) because of an injury.
Foul by Álvaro Morata (Chelsea).
Adrián (West Ham United) wins a free kick in the defensive half.
Corner, Chelsea. Conceded by Pedro Obiang.
Attempt blocked. Eden Hazard (Chelsea) left footed shot from a difficult angle on the left is blocked. Assisted by Cesc Fàbregas.
Attempt blocked. Gary Cahill (Chelsea) right footed shot from outside the box is blocked.
Pedro Obiang (West Ham United) is shown the yellow card for a bad foul.