Matheus Pereira and Callum Robinson each scored doubles after Thiago Silva was sent off as West Brom secured a stunning 5-2 Premier League win at Chelsea.
The Blues were unbeaten in 14 games under head coach Thomas Tuchel and had not conceded a goal in seven matches, but they suffered a shock defeat at the hands of the second-bottom Baggies at Stamford Bridge on Saturday.
Christian Pulisic put Chelsea in front, but Silva was given his marching orders for a second bookable offence only 29 minutes into his return from injury.
The clinical Pereira then struck twice in first-half stoppage time and things went from bad to worse for Chelsea after the break, with Robinson and Mbaye Diagne giving Albion a three-goal advantage.
Mason Mount pulled one back, but Robinson applied the finishing touches in injury time as the brilliant Baggies secured a first away league victory over the London club since 1978 to raise hopes they can pull off an unlikely great escape.