Christian Pulisic's first Chelsea goal for Thomas Tuchel was not enough to save the German from the first defeat of his managerial tenure as West Bromwich Albion won 5-2 at Stamford Bridge to revive their hopes of Premier League survival.
When Pulisic turned home the rebound from Marcos Alonso's free-kick to give Chelsea a 27th minute lead it seemed the Blues were destined for another victory under Tuchel, whose side have been defined by defensive solidity since his appointment. But having held the likes of Atletico Madrid, Liverpool and Manchester United goalless in a run of two conceded in 14 games the rearguard collapsed after Thiago Silva was sent off a third of the way through the game.
The game turned on Silva's clumsy foul on Okay Yokuslu, earning him a second yellow card and offering renewed hope to West Brom, who now sit seven points from safety having played a game more than 17th placed Newcastle United. It would be one of the Premier League's great escapes if Sam Allardyce's side were to escape the drop but on the basis of their dismantling of Chelsea's 10 men there is quality to get results in their remaining eight games.
Matheus Pereira was the star of the show, ghosting through Chelsea's high line to lob the ball over Edouard Mendy in the second of five minutes of stoppage time. The Brazilian was not done with his first half evisceration of his hosts, holding off the defence and sliding a low shot through several bodies and into the bottom corner.
Tuchel had responded to Silva's sending off by withdrawing his attacking midfielders, first withdrawing Hakim Ziyech for Andreas Christensen soon after the red card and then with Mason Mount replacing USA international Pulisic at the interval. Though Chelsea offered some attacking punch early on in the second half it came at the cost of acres of space to be countered into. The outstanding Darnel Furlong, whose back post clearance had denied Alonso early in the game, exploited those gaps quite excellently in a flowing West Brom break, steering his cross to the edge of the box for Callum Robinson to thump home a powerful effort.
When Connor Townsend's backheel and Pereira combined to tee up Mbaye Diagne for a fourth it seemed that Chelsea were doomed to their first defeat since January 19 but in the 71st minute Timo Werner squared for Mount to tap into an empty net, setting the stage for the 10 men to push West Brom ever deeper. Despite having lost both starting center back Darra O'Shea and his replacement Branislav Ivanovic to injury the Baggies stood firm, Robinson netting the fifth in stoppage time, and earned three points that Allardyce will hope sets the stage for the remarkable run of results required to secure Premier League safety.
Chelsea, meanwhile, will drop out of the top four if West Ham United win at Wolverhampton Wanderers and could also drop behind Tottenham if they pick up three points. Liverpool and Everton will also be pushing to close the gap to the Blues over the coming days.