Ireland international Callum Robinson scored on the double as Relegation-threatened West Bromwich Albion gave their Premier League survival hopes a huge boost with a stunning 5-2 victory over 10-man Chelsea at Stamford Bridge.
Chelsea took the lead just before the half-hour mark when Marcos Alonso's free kick came off the post and Christian Pulisic scored from the rebound, the American winger's first goal under manager Thomas Tuchel.
Two minutes later Chelsea defender Thiago Silva was sent off after receiving a second yellow card for a reckless challenge.
West Brom used their numerical advantage to good effect as Matheus Pereira scored two goals in quick succession in first-half stoppage time to put the visitors ahead at the interval.
They added two goals on the counter-attack five minutes apart in the second half, with Callum Robinson volleying home a Darnell Furlong cross before Pereira turned provider for Mbaye Diagne.
Chelsea pulled one back to make it 4-2 when Timo Werner set up substitute Mason Mount for a simple tap-in but West Brom scored their fifth in stoppage time when Robinson found space and chipped onrushing keeper Edouard Mendy.
West Brom remained second-bottom of the table, seven points from the safety zone.
Silva's red card undoubtedly changed the course of this match but West Brom deserve huge plaudits for both their tactical discipline and clinical attacking play.
Chelsea's slow start was eased by Pulisic's smart reaction to rush first to the loose ball and tap home after Marcos Alonso's free-kick.
Alonso's curling effort struck the post but Pulisic stormed in and slotted into the net to put the Blues ahead.
Spain full-back Alonso could so easily have put Chelsea ahead moments earlier when Darnell Furlong headed clear under pressure as he lurked behind him ready to pounce at the far post.
Just as Chelsea looked to cement their control, however, their situation crumbled.
Silva's big return was over inside half an hour as he was sent off for a second yellow card.
In a near carbon copy of his first booking, Silva was dismissed when battling to recover from a loose Jorginho pass.
Both times the Brazil veteran had too much to do and wound up sliding in late - but the second yellow for his challenge on Okay Yokuslu certainly appeared harsh.
Hakim Ziyech was the luckless fall guy for the red card, sacrificed and subbed off to bring defender Andreas Christensen into the action.
Chelsea switched to a 4-4-1 with Reece James on the wing, Pulisic on the left and Timo Werner up front on his own.
While Chelsea were caught up with recalibrating, though, the Baggies busied themselves with turning the tables.
Goalkeeper Sam Johnstone's raking pass bisected the new-look Blues defence and Pereira raced through, controlled and then lobbed the stranded Edouard Mendy to put the visitors level.
In a flash Pereira was on the scoresheet again, keeping his cool and sorting out his feet in the area before sliding home another smart finish.
The half-time whistle handed Chelsea a chance to draw breath and further tweak their new set-up, with the visitors leading 2-1.
Mason Mount replaced Pulisic at the break, with the dependable academy graduate having only been on the bench after three straight matches with England during a draining international break.
Alonso's low effort was deflected onto the post to open the second half after Johnstone had flapped at James' hoisted cross.
Mateo Kovacic saw a drive deflected over the bar, then Kurt Zouma hooked high and wide from a corner.
Chelsea's designs on restoring parity evaporated when Furlong capped a fine West Brom move with a dangerous cross which Robinson turned home in style in the 63rd minute.
Five minutes later, Diagne drilled home a fourth as the 10-men hosts started to unravel but then all of a sudden Mount slotted home to make it 4-2.
Werner could - and should - have taken on Johnstone and scored but opted instead to play in Mount, who made no mistake.
The last word belong to West Brom, however, with Robinson on target again, lifting over Mendy for his second of the day in stoppage time.