Mohamed Diame ratified a rousing second-half performance by West Ham as he came off the bench to score his side's second goal in a breathless 3-1 win over troubled London foes Chelsea at Upton Park.
Chelsea were the dominant side in the first period after Juan Mata struck the opening goal on 11 minutes, but astonishingly fell apart in the second period as goals from Carlton Cole (63), Diame (86) and Modibo Maíga (90+1) condemned Rafael Benitez's side to another horrid day at the office.
Benitez has failed to win any of his opening three games with this loss accompanying two draws since replacing the sacked Roberto Di Matteo as manager of Chelsea while his new side have not won in the Premier League for seven matches - the club's worst run in 18 seasons.
The three points handed West Ham fans a much-needed boost after the news emerged before kick-off that striker Andy Carroll had been ruled out for six to eight weeks with a knee injury.
Benitez saw banners appear from the unhappy visiting fans calling for his head at full-time, but must be wondering how this afternoon went so wrong.
He appeared to be heading for his first win as manager when Mata finished off a superb pass from Fernando Torres on 11 minutes after Victor Moses had managed to pick out the much-maligned striker on the run.
His pass to Mata - back from a hamstring injury - was a thing of real beauty with the former Valencia player doing the rest to convert his fifth goal of the season in the Premier League.
Chelsea were almost in for a second goal on 21 minutes as Guy Demel had his pocket picked by Ramires.
He managed to seek out Moses, who failed to connect properly with the pass from 12 yards as the shot wandered wide.
West Ham were strangely subdued as Chelsea's passing game continued to hold sway.
But Kevin Nolan was perhaps unfortunate to see a goal disallowed with James Collins deemed to be pushing Branislav Ivanovic in winning a free-kick in the air.
Chelsea continued to have few problems finding gaps in the home defence with Jussi Jaaskelainen diving to claw out a Mata shot on 39 minutes after some more fine play by Moses to play provider. Joey O'Brien was also alert to make another fine tackle on Mata from the rebound.
The half finished with Petr Cech booked for handling outside of his area before making amends to swipe a Nolan header over the bar.
West Ham were utterly transformed in the second half. Diame was a workhorse of considerable class when he replaced Gary O'Neil at half-time with Matthew Taylor taking over from James Tomkins.
Diame tore into challenge after challenge as Chelsea's midfield proved to be lightweight in the face of some severe heat.
West Ham equalised when a Cole goal was allowed to stand despite him clearly fouling Ivanovic in the air from a deflected Matt Jarvis cross. It was good fortune, but you could hardly argue with the scoreline.
Chelsea almost regained the lead when Torres drove the ball straight at Jaaskelainen after he had repelled an effort from the disappointing Eden Hazard.
Mata then saw a free-kick from 25 yards out come back off the inside of a post as it suddenly became apparent it was not going to be the visiting side's day.
But West Ham continued to prompt moments of anxiety as Cech saved from Jarvis before Ashley Cole nodded a Winston Reid header off his own line.
The home side moved 2-1 to the good when Diame drilled the ball beyond Cech from 12 yards out after some fine play by Carlton Cole in picking him out.
West Ham were not finished and manager Sam Allardyce celebrated like he had won the Premier League with his coaching staff when Ashley Cole shed possession under pressure in injury time.
Cech saved from Taylor, but the ball broke kindly for Maiga to finish off the match and finish off Chelsea for the day.
MAN OF THE MATCH
Mohamed Diame (West Ham United) - A devastating display of power and considerable class in the second half. Chelsea needed a man like Diame in the second half as they came up well short in the face of such pressure.
West Ham United: Jaaskelain 7, Demel 6, Collins 6, Reid 7, O'Brien 8, Noble 7, Nolan 8, Tomkins 5, Jarvis 7, O'Neil 5, Cole 7..subs: Maiga 7, Taylor 7, Diame 9
Chelsea: Cech 7; Azpilicueta 6, Ivanovic 6, Cahill 6, Cole 5; Mikel 5, Ramires 5; Moses 6, Mata 7, Hazard 5; Torres 5..subs: Marin 6, Oscar 6