David Moyes recorded a vital first victory as West Ham manager to ease the Hammers' relegation fears and effectively end Chelsea's title hopes.
In the day's early kick-off, the Irons were rewarded for a fast start when Marko Arnautovic broke the deadlock on six minutes following a neat passing move.
Chances were at a premium for both sides following the early goal, as West Ham defended resolutely to take local bragging rights.
Here's what we learned from the encounter at the London Stadium.
Slow starts become a real worry for Chelsea
Despite their excellent run of form that saw six wins and one draw in their previous seven heading into Saturday's early kick-off, it's not all been totally plain sailing for Antonio Conte's men.
Slow starts, in particular, have been a constant theme in recent months, with Marko Arnautovic's 6th minute opener serving as the fourth time that the Blues have conceded the first goal in their last five matches.
Worryingly, they were once again caught cold here. All around the pitch, West Ham players took the ascendancy over their opposite numbers. Second-balls were won by a dominant home midfield, while a well-drilled backline left Chelsea's attackers playing the majority of the early stages with their backs to goal. Put simply, it looked as though the hosts wanted it that bit more in the opening stages.
This, though, was not just an exercise in physicality and aggression. With Lanzini demanding the ball and forwards Arnautovic and Antonio driving at Chelsea's defence, the hosts played more than their fair share of good football, too. Indeed, the move for the Hammers' opener evidenced a side growing in confidence in possession, as Lanzini and Arnautovic produced an incisive move with such a tangible end product.
For at least 45 minutes, there was only one team in control at the London Stadium. And that, given the respective ambitions of the two teams, must be a cause of real concern for Conte.
Moyes continues to draw positive response to Everton thrashing
Humbled by former club Everton at the end of last month and with Manchester City and Chelsea next up, David Moyes could have been forgiven for wondering what he'd got himself into at the London Stadium.
Few, if any, would have banked on the Hammers even being competitive against the high-flying duo- but in adversity, Moyes and his players have come out fighting.
Important steps were taken in the 2-1 defeat to Manchester City, who were taken to the death despite starting as favourites. Defensively, the east Londoners appeared significantly stronger, while there was also a unity apparent not seen at Goodison Park the previous week. Up front, Michail Antonio has been a revelation in the lone striking role. This was a team, and not a group of individuals, for perhaps the first time this season.
The upturn in fortunes continued here, too, as the hosts fought for every scrap against the Premier League champions. As far as responses go, it's probably about as impressive as it gets given the low base West Ham have come from.
Even if performances haven't always been rewarded with the points they deserve, Moyes' West Ham will stand a fighting chance of getting out of danger if they continue in the same vein.
Chelsea title hopes suffer significant blow
11 points off top spot ahead of kick-off, Chelsea knew three points were essential in east London if they were to hang onto leaders Manchester City's coattails and keep the title race alive.
Now, following defeat at the London Stadium, there's a chance for City to open up a 14-point gap between themselves and the champions. That Chelsea are so far behind Guardiola's men says as much about the Blues' deficiencies as it does the superb football being played at the Etihad.
As it stands, it appears as though they've been left behind by others due to an underwhelming summer of recruitment. For while Alvaro Morata's addition has been a clear success, the likes of Davide Zappacosta and Tiémoué Bakayoko are yet to hit the ground running at all. The latter two were particularly dismal here, with full-back Zappacosta regularly beaten by Arthur Masuaku and the clumsy Bakayoko hooked at half-time. Neither player seems to be an upgrade on Victor Moses or Nemanja Matic.
The case could be made that, for long spells, this was among Chelsea's worst performances of the season. They may have enjoyed close to 70% possession and twice as many shots on goal as their hosts, but there was no threat of note until the closing stages. By then it was almost too late. Conte's men were restricted to playing in certain less dangerous areas and taking pot-shots from distance.
Rather than assert themselves upon proceedings, they were managed rather comfortably by Moyes' improving side. Ahead of Sunday's Manchester derby, defeat could well be fatal as far as their hopes of retaining last season's title are concerned.
Hart's World Cup hopes in jeopardy
The writing was on the wall for Joe Hart. Ineligible for last Sunday's game against Manchester City, Adrian's impressive outing in goal left David Moyes with a big decision to make for today's fixture.
Rhetoric-wise, the England stopper should have known what was coming. "I don't want anyone to feel they have a divine right to play," Moyes told reporters at his pre-match press conference. "I'm not going to reveal just at this moment which of them will be playing on Saturday. I am also aware that Joe is England's first-choice goalkeeper and there is a World Cup next summer but, as West Ham's manager, I am only interested in winning matches and doing what is right for this club."
Poor form has made Hart an expendable presence- he was at fault for a couple of Everton goals and has conceded the most in the division- and, as at the Etihad, Adrian seized his chance again. There should be no doubt as to who keeps goal for the Hammers on Wednesday against Arsenal.
With the games coming thick and fast and next summer's World Cup drawing into focus, Hart now faces a real battle to hang onto his Three Lions berth. Waiting in the wings are two young, talented goalkeepers in Jordan Pickford and Jack Butland who will both look to press their own claims as their rival sits on the sidelines. Who would bet against at least one of the duo forging ahead?
For club and country, Hart risks getting left behind- and the feeling lingers that he only has himself to blame.
