The previously miserly Blues saw Thiago Silva sent off on his return from injury on Saturday – with that dismissal sparking a remarkable meltdown after Chelsea had initially led through Christian Pulisic.
Man of the match Matheus Pereira struck twice in quick succession for West Brom shortly before half-time, while there were further second-half efforts from Callum Robinson and Galatasaray loanee Mbaye Diagne before substitute Mason Mount’s goal gave the hosts a brief lifeline.
However, Robinson netted again in stoppage time to complete the rout of a Chelsea team that had conceded just twice in 14 previous matches across all competitions under Tuchel and kept seven successive clean sheets.
Chelsea’s first defeat since the 2-0 loss at Leicester under Frank Lampard on January 19 means they can now be leapfrogged by both Tottenham and West Ham in the coming days, if their London rivals manage to win at Newcastle and Wolves respectively.
Meanwhile, a first victory in four games for West Brom – their first at Stamford Bridge since 1978 – leaves Sam Allardyce’s side seven points adrift of safety with eight matches left to play this term.