James Ward-Prowse set up two goals on his West Ham debut and Lucas Paqueta scored a late penalty as the 10-man Hammers somehow secured a 3-1 victory over Chelsea.
Ward-Prowse, the £30m signing from Southampton, set up goals for Naif Aguird and Michel Antonio, the equalizer for Carney Chukwuemica.
But Mauricio Pochettino’s intensely assembled side will wonder how they walked out of the London Stadium empty-handed.
Enzo Fernandez missed a penalty and West Ham were forced to play the last 25 minutes with a man-less lead after Agwerd saw red.
However, not only did David Moyes’ side cling on for their first win of the season, they sealed it in injury time with a volley from Paqueta.
Ward-Prowse is famous for his set-piece ability but certainly not even Moyes expected such an immediate impact.
And seven minutes after swooping in from the second corner in his purple and blue shirt, Ward-Prowse swung in a cross that Aguirre nodded unmarked at the far post.
Chelsea have now conceded a goal in each of their last 13 matches, their worst since 1996.
But they were working their way up the other end with Nicholas Jackson keen to get behind the West Ham backline.
One such kick and slash clipped in, but when Ben Chilwell returned to the area his cross was cleared by Kurt Zouma only as far as Chukwuemica.
And the 19-year-old established himself before firing the ball past Alphonse Areola for his first goal for Chelsea.
Paqueta was in the West Ham squad despite being investigated by the Football Association for a possible breach of betting rules.
The Brazilian midfielder is said to have been shocked by the investigation, which he believes revolves around bets placed in his home country on yellow cards awarded against him.
Paqueta was already booked in the first half for an interception after complaining about a corner being awarded, but moments later he was inches away from scoring, dragging the ball down into a crowded penalty area only to clip a post.
Chelsea had a chance to go ahead just before half-time after Tomas Soucek edged Raheem Sterling in the area, but Fernandes’ penalty was saved by Areola.
Instead, Antonio fired West Ham into the lead again eight minutes into the second half when Ward-Prowse’s slick ball raced over the top.
Pochettino turned on Moises Caicedo, Chelsea’s latest record signing, for his debut just after the hour mark, as the Ecuadorian promptly sent a long-range shot wide of the goal.
West Ham were then reduced to 10 in the 68th minute after Aguird received a second yellow card for an unnecessary foul on Jackson.
But Chelsea, it seemed, had run out of ideas and could only muster a deflected shot from Noni Madueki that was fired wide by Areola.
The away fans left en masse even before Paqueta sealed the win with a penalty after Caicedo fouled Emerson Palmieri.