Victor Moses and Diafra Sakho ended Manchester City’s 11-match run of consecutive Premier League victories as West Ham recorded a stunning 2-1 success at Eastlands.
Without injured centre-half and captain Vincent Kompany, City conceded goals for the first time this season and looked uncharacteristically vulnerable in defence.
After West Ham’s first half strikes, a goal from Kevin De Bruyne just before the interval gave City hope of salvaging a point and set up a second half of non-stop pressure from the hosts.
City came into the contest having not conceded a goal in the previous 566 minutes of English Premier League football but took just six minutes to allow West Ham to score, and a further 25 to let in a second.
It was no more than Slaven Bilic’s side deserved as they demonstrated the same organisation, commitment and speed on the counter-attack that had already seen them record unexpected victories at Arsenal and Liverpool this season.
The home side had already threatened with an early header from Eliaquim Mangala which was punched clear by Adrian, by the time West Ham took the lead through Moses, the winger on loan from Chelsea and playing just his second game for the visitors.
Inexplicably, City failed to close down Dmitri Payet on the edge of their penalty area and, even more unforgivably, also neglected to pressure Moses as he collected a sideways pass, gathered himself and deposited a 20-yard shot past Joe Hart into the City net.
The Blues had conceded two goals in the previous 821 minutes of league football but would soon concede a second to Bilic’s irrepressible young team.
Just after the half hour, Payet’s disputed right-wing corner was headed goalwards by Winston Reid, Fernandinho was slow to clear and Pedro Obiang was able to steer the ball across goal for Sakho to hook in, under pressure.
It was a stunning turn of events and West Ham were clearly not satisfied to sit on a two-goal cushion.
Debutant Nicolas Otamendi was forced to make a crucial block to prevent Carl Jenkinson’s pull-back from finding Sakho.
Then Mangala made an expert tackle to deny Sakho what would have been another goalbound shot just before the break.
That second defensive play proved all the more important as City halved their deficit in first half injury-time through De Bruyne, who was making his full debut.
Sergio Aguero’s excellent control in midfield forced three defenders to converge upon him and allowed him to find the unmarked De Bruyne whose shot from the edge of the area was as accurate as that of Moses.
City had lost the influential and in-form David Silva to injury in the pre-game warm-up – his place going to Jesus Navas – but still posed plenty of attacking threat.
Adrian parried a powerful Navas strike, Aguero curled a shot just wide and the West Ham keeper then made an important double save from Aguero.
After the interval, the home side made their predictable cavalry charge on goal.
Yaya Toure, in a more advanced role, was central to many of their early second-half forays, threatening the West Ham goal four times in the first 10 minutes alone.
After 63 minutes, Toure’s one-man assault on the east London side’s goal finally looked like paying dividends when he found space inside the area to collect a Demichelis pass but dragged a low shot just wide.
De Bruyne played in Navas, whose cross was well defended, and Toure had a sight of goal only to see his shot blocked, this time by Mark Noble.
Bilic tried to stem the tide by moving Sakho back into midfield but the flow of traffic remained relentless and almost exclusively in his team’s direction with Adrian being required to make a magnificent 85th-minute fingertip save to keep out Otamendi’s header, from a De Bruyne corner.
City remain top of the table on 15 points from six matches, but the result meant West Ham surged up into second place on 12 points.