Match ends, Manchester City 1, West Ham United 2.
Manchester City missed the chance to move six points clear at the top of the Premier League as West Ham produced a clinical counter-attacking display at Etihad Stadium.
Victor Moses drilled in his first West Ham goal with a low 20-yard shot which crept inside Joe Hart's left-hand post.
The visitors doubled their lead when Diafra Sakho hooked in from six yards.
City's record signing Kevin De Bruyne fired in to halve the deficit, but West Ham held on to move into second place.
Yaya Toure's angled shot whistled past the Hammers' post after the break, while away keeper Adrian made a string of excellent saves.
City dominated the game, controlling 72% of possession and attempting 27 shots at goal, but were unable to find a way past an away side who are now three points behind them.
Hammers happy on the road
Many West Ham fans might have been forgiven for fearing the worst when the fixture computer handed their team trips to Arsenal, Liverpool and Manchester City in their opening three away games.
City had not conceded a single Premier League goal before Saturday's match, but without injured skipper Vincent Kompany were overcome by defensive uncertainty.
Left-back Aleksandar Kolarov backed off to allow Moses to shoot for the opener, while the home defence reacted slowly to a set-piece as Sakho pounced.
City's European hangover
Manuel Pellegrini's side suffered a chastening midweek defeat when they surrendered a 1-0 lead against Italian side Juventus to lose 2-1 in their Champions League opener.
But the Blues, who netted 11 goals without reply in their opening five top-flight matches, would have been confident of bouncing back against opposition with a woeful record at Etihad Stadium.
Going into the match, the Hammers had suffered seven consecutive defeats in east Manchester, scoring just three goals and conceding 20 in the process.
However, Slaven Bilic's side executed their game plan perfectly, catching their opponents on the break, taking the chances they created, and then defending stoutly to preserve their lead.
Reid shines in heart of defence
While Spanish keeper Adrian will take most of the plaudits for keeping out City, Hammers centre-back Winston Reid was also instrumental in helping his team clinch their fourth win of the Premier League season.
The New Zealand international made the most clearances (21) and interceptions (four) during the match, while he also won back possession for his side (nine) more than any other player.
|Match of the Day commentator Steve Wilson|
|"Winston Reid might have only cost £3m from Danish side Midtjylland but he has just produced one of the best individual defensive displays that I have ever seen."|
Stats you need to know
- Victor Moses' strike was the first league goal Manchester City had conceded in 572 minutes.
- West Ham's Dimitri Payet has contributed to 12 goals (four goals, eight assists) in his last 14 league appearances (Premier League and Ligue 1 combined).
- Only three other sides have recorded away wins at Arsenal, Liverpool and Manchester City in the same Premier League season - Everton (1995-96), Manchester United (2004-05) and Chelsea (2005-06).
- Diafra Sakho has scored in both Premier League games he has played against City.
Man of the match - Winston Reid
Manchester City manager Manuel Pellegrini: "We played very well in the second half, though not in the first. We were not the aggressive team we need to be at home.
"We corrected those things, played very well, created a lot of chances but didn't score. We were under pressure, falling two goals behind.
"We knew this would be a difficult game because it was not luck that meant they beat Liverpool and Arsenal. We needed to be better with our finishing."
West Ham manager Slaven Bilic: "You need a bit about you if you want to score two goals here, or two goals at the Emirates or three at Anfield.
"You've got no chance unless you sprint until the end of the game. We deserved it, considering the way we played in the first half and fought in the second.
"But this was hard - City pressed us and City were the better team in the second half."
What's next for both teams?
The League Cup. City, who could make a raft of changes, go to Sunderland on Tuesday for their third-round tie. West Ham are also playing away in another all-Premier League tie at Leicester.
Both teams return to top-flight action on 26 September when City travel to Tottenham and the Hammers host Norwich.
- 20MangalaSubstituted forDemichelisat 45'minutes
- 11KolarovBooked at 54minsSubstituted forIheanachoat 84'minutes
- 42Y Touré
- 17De Bruyne
- 7SterlingSubstituted forBonyat 66'minutes
- 12JenkinsonSubstituted forCollinsat 85'minutes
- 14ObiangBooked at 45mins
- 20MosesSubstituted forAntonioat 60'minutes
- 28LanziniBooked at 22minsSubstituted forJelavicat 69'minutes
- Robert Madley
- Shots on Target
Second Half ends, Manchester City 1, West Ham United 2.
Offside, West Ham United. Pedro Obiang tries a through ball, but Nikica Jelavic is caught offside.
Attempt blocked. Wilfried Bony (Manchester City) header from the centre of the box is blocked. Assisted by Yaya Touré with a cross.
Fernandinho (Manchester City) wins a free kick on the right wing.
Foul by Michail Antonio (West Ham United).
Corner, Manchester City. Conceded by Michail Antonio.
Kelechi Iheanacho (Manchester City) wins a free kick in the defensive half.
Foul by Aaron Cresswell (West Ham United).
Corner, West Ham United. Conceded by Joe Hart.
Attempt saved. Dimitri Payet (West Ham United) right footed shot from outside the box is saved in the bottom left corner.
Foul by Yaya Touré (Manchester City).
Nikica Jelavic (West Ham United) wins a free kick in the defensive half.
Corner, Manchester City. Conceded by Adrián.
Attempt saved. Nicolás Otamendi (Manchester City) header from the left side of the six yard box is saved in the top centre of the goal. Assisted by Kevin De Bruyne with a cross.
Substitution, West Ham United. James Collins replaces Carl Jenkinson because of an injury.
Substitution, Manchester City. Kelechi Iheanacho replaces Aleksandar Kolarov.
Corner, Manchester City. Conceded by James Tomkins.
Attempt blocked. Kevin De Bruyne (Manchester City) left footed shot from outside the box is blocked. Assisted by Yaya Touré.
Attempt blocked. Yaya Touré (Manchester City) right footed shot from the right side of the box is blocked. Assisted by Jesús Navas.