Relegation-threatened West Bromwich Albion secured a stunning 5-2 win over 10-man Chelsea at Stamford Bridge on Saturday to hand Thomas Tuchel his first loss as the London club's manager.
West Brom, second-bottom of the table, moved seven points from the safety zone while Chelsea stayed fourth but could be overtaken by West Ham United and Tottenham Hotspur this weekend.
Chelsea were looking to record an eighth consecutive clean sheet but Thiago Silva's sending-off for a second yellow card after a reckless challenge opened the floodgates as West Brom ran riot.
"There's no need to concede five goals when you have our quality," Tuchel told BBC Sport after Chelsea's worst home defeat in nearly 10 years.
"We could not adapt to the situation, which is a surprise as we were leading. We're normally confident enough to defend but we were sloppy and kept making big mistakes. We were punished hard and it's tough to digest."
Chelsea took the lead two minutes before Silva's red card when Marcos Alonso's free kick came off the post and Christian Pulisic scored from the rebound, the American winger's first goal under Tuchel.
West Brom used their numerical advantage to good effect as Matheus Pereira scored two goals in quick succession in first-half stoppage time -- which included an assist from goalkeeper Sam Johnstone -- to put the visitors ahead at the interval.
They struck twice 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 Mason Mount for a simple tap-in but West Brom scored their fifth in stoppage time when Robinson chipped onrushing keeper Edouard Mendy.
"People will always look to the fact they went down to 10 men but I thought we started the game brightly and created chances. I thought Thiago Silva was lucky to stay on after the first foul," West Brom manager Sam Allardyce said.
"Our goals were all quality goals, not just the finishing but the build-ups. It's just a bit of a shame we didn't show that quality in some others games but this still gives us a bit of hope."