West Ham United picked up their first win under David Moyes by upsetting Premier League champions Chelsea 1-0 at the London Stadium this lunchtime.
Marko Arnautovic's early strike stopped the Irons' winless top-flight run at eight and dealt the Blues another hammer blow in their title defence.
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West Ham started brightly and took the lead on six minutes through Arnautovic's first goal since his big-money arrival from Stoke City in the summer.
The Austrian exchanged a quick one-two with Manuel Lanzini on the edge of the box, spun away from Andreas Christensen and placed a left-footed shot into the bottom corner.
Alvaro Morata felt that he should have had a 14th-minute penalty for a shirt tug by Winston Reid, but the rest of the first half passed without much worry for the Hammers.
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Having been preferred to Joe Hart in goal, Adrian was called upon to make routine saves from N'Golo Kante and Davide Zappacosta as Chelsea began to push the hosts back.
The in-form Eden Hazard was kept quiet in the opening 45 minutes aside from one half-chance when he dragged a left-footed volley wide from 12 yards out.
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West Ham had their own penalty shout turned down early in the second half as Arnautovic knocked the ball against the hand of the unknowing Christensen.
Antonio Conte made all three of his substitutions before the 65-minute mark, but none of Pedro, Victor Moses or Willian made them a greater attacking threat.
It was not until the final 10 minutes that Chelsea really pushed for an equaliser, with Morata missing their clearest opportunity seven minutes from time.
Having been played onside by the otherwise-excellent Arthur Masuaku, the unmarked Spaniard rushed his shot and fired into the side-netting from 12 yards.
Hazard came close soon after with a rising left-footed shot, but Chelsea sunk to a first league defeat since October, and fifth London derby loss already of Conte's tenure.
The Blues could end the weekend 14 points behind Manchester City at the top of the table, while West Ham remain in the relegation zone despite this morale-boosting win.
West Ham (5-3-2): Adrian; Zabaleta, Reid, Ogbonna, Cresswell, Masuaku; Obiang, Noble, Lanzini; Arnautovic (Sakho 69'), Antonio (Ayew 79')
Chelsea (3-5-2): Courtois; Azpilicueta, Christensen, Cahill; Zappacosta (Willian 64'), Kante, Bakayoko (Pedro 45'), Fabregas, Alonso (Moses 55'); Hazard, Morata
