Mauricio Pochettino remains winless after two Premier League games as Chelsea manager as 10-man West Ham United defeated their London rivals 3-1 on Sunday afternoon.
A Nayef Aguerd header propelled the Europa Conference League winners into an early lead, but a fine Carney Chukwuemeka finish drew the Blues level before Alphonse Areola kept out Enzo Fernandez's spot kick.
Michail Antonio restored the Hammers' advantage before Aguerd was sent for an early bath, but Chelsea never looked like breaking down a dogged West Ham defence, and the hosts added a third deep into injury time via Lucas Paqueta's penalty.
While £115m man Moises Caicedo was only on the bench for Chelsea, West Ham handed a debut to James Ward-Prowse, and the Englishman took just seven minutes to light up the London Stadium with his dead-ball excellence.
After one corner led to a Kurt Zouma header being kept out by Robert Sanchez, Ward-Prowse's second delivery was on the money for Aguerd, who easily outmuscled Conor Gallagher and nodded home into the bottom corner past a stranded Sanchez.
The Blues responded well to that early setback, and the rapid Nicolas Jackson beat Areola to the ball inside the area in the 10th minute, before being brought down by the onrushing Frenchman.
However, Jackson had just gone slightly too early, and the offside flag spared Areola's blushes, but there was nothing the Frenchman could do about a deserved leveller for Chelsea in the 28th minute.
Jackson and Ben Chilwell both saw passes from the left byline cut out by the West Ham backline, but Zouma could only play the ball out to Chukwuemeka, who took Tomas Soucek out of the game with a sharp turn inside before curling home into the bottom corner with aplomb.
Chelsea's forward trio were wreaking havoc on the West Ham backline, and Raheem Sterling won the visitors a penalty late on in the first half, as Soucek clattered into the Englishman right on the line.
Fernandez took on the responsibility from 12 yards, but the Argentine's spot kick was a good height for Areola, who dived to his right to deny the Argentine as West Ham somehow went into the break on level terms.
However, what should have been a memorable afternoon for Chukwuemeka was cruelly cut short by an apparent knee injury, and Mykhaylo Mudryk came on to replace the former Aston Villa youngster for the second half.
Antonio had been an anonymous figure during the first half, but just eight minutes into the second, the 33-year-old proved his worth to David Moyes with a thunderous finish to propel the hosts back into the lead.
Axel Disasi was caught up in possession near the halfway line, and Ward-Prowse - bagging a second assist on his debut - lifted a long pass over the top for Antonio, who was forced slightly wide by Disasi but still found the far corner with a ferocious strike.
The hosts had a couple of half-chances to notch a third, but first-half goalscorer Aguerd went from hero to zero in the 68th minute, as he caught Jackson with a late challenge and was sent off for a second bookable offence.
Chelsea spent the remainder of the match camped in the West Ham half, but a succession of crosses were easily beaten away by the hosts, while Mudryk had a decent chance to trouble Areola with a volley at the back post in the 85th minute, but he got his connection horribly wrong as the ball sailed towards the sideline.
A chaotic six-minute injury time period firstly saw Pablo Fornals denied by the feet of Sanchez, before his opposite number Areola produced a wonderful full-stretch save from Noni Madueke's deflected strike, which would have nestled into the bottom corner otherwise.
While Ward-Prowse enjoyed a dream debut, substitute Caicedo endured a nightmarish start to life in blue, as the Ecuadorian scythed down ex-Chelsea man Emerson Palmieri in the penalty area with 95 minutes on the clock, and Paqueta put the final nail in the coffin by sending Sanchez the wrong way.
West Ham's third Premier League game of the season takes them to Brighton & Hove Albion next Saturday, while the Blues seek to bounce back when they host Luton Town on Friday.
