One of the biggest shocks of the season was recorded in Saturday's lunchtime kickoff in the Premier League, as 19th-placed West Bromwich Albion beat Chelsea 5-2 at Stamford Bridge.
The hosts started with a three-man defence, with Thiago Silva making his first start in eight weeks, while Mason Mount dropped to the bench. Chelsea expectedly started the game on the front foot, with West Brom sitting back and absorbing the pressure in the early stages.
However, for all the pressure exerted by the hosts, they failed to create too many clear-cut chances, as a resolute Baggies defence kept them at bay.
Sam Allardyce was forced into an early change when Dara O'Shea went down injured and had to be replaced by former Chelsea man Branislav Ivanovic in the 24th minute.
Chelsea finally got the lead their pressure deserved when Christian Pulisic turned home after Marcos Alonso's well-struck free-kick rebounded off the post. Their joy, however, lasted less than three minutes, as Thiago Silva was sent off for an early shower after two quick bookable offences.
Thomas Tuchel reacted to Silva's red card by sacrificing Hakim Ziyech for Andreas Christensen. West Brom themselves had to make an injury-enforced change, with substitute Branislav Ivanovic going off after pulling his hamstring.
Nevertheless, the visitors smelled blood with their numerical advantage and pushed forward in the dying stages of the first half.
They bagged their equaliser when Sam Johnstone's routine punt upfield bypassed the Chelsea defence and allowed Matheus Pereira to finish past Edouard Mendy with an exquisite lob.
Chelsea struggled to cope with the high press deployed by Allardyce's men as West Brom hit the post through Matt Philips before deservedly going ahead a minute later. Once again, the hosts struggled to clear their lines, and some excellent footwork in the area saw Pereira wrong-foot Mendy to put West Brom 2-1 up.
Tuchel made a half-time substitution, with the in-form Mason Mount coming on for Christian Pulisic, and Chelsea asked all the questions early in the second half.
Marcos Alonso hit the bottom of the left post after a quick break in the 54th minute, but that was as good as it got for Chelsea.
Matheus Pereira had an excellent chance to get his hat-trick after he found space on the edge of the area. But his shot was hit straight at Mendy, and the scoreline remained what it was.
West Brom went 3-1 up less than a minute later. Darnell Furlong sent a pristine cross into the Chelsea box, and first-half substitute Callum Robinson was on hand to convert with an exquisite first-time volley.
The visitors remarkably took a three-goal lead five minutes later when an excellent counterattack saw Mbaye Diagne find the bottom corner from 12 yards.
It was one of the best team goals in recent memory and was no less than West Brom deserved for their impressive start to the second half.
Chelsea pulled a goal back after Timo Werner set Mason Mount up to tap into an empty net. The fact that the German elected to take several touches rather than shoot first-time underlined his frail confidence.
The game continued in an end-to-end fashion, and the visitors got a fifth goal when Callum Robinson bagged his brace with a delightful lob in injury time.
Chelsea's defeat opens up the race for the top four in what has been an intriguing 2020-21 Premier League season. On that note, let's have a look at five talking points from the game.
#1 Thiago Silva produces a nightmare performance against West Brom
Thiago Silva had been sidelined for seven weeks with a thigh injury, but he has earned plaudits for his animated support for the team from the stands.
After returning to full fitness, the 36-year-old was drafted straight to the Chelsea starting lineup against West Brom. But Silva put out one of the worst displays of his long and illustrious career.
A shaky start saw him give away the ball on several occasions. He was shown his first booking for a foul on compatriot Mateus Pereira as early as the fifth minute.
That set the tone for what was to follow. Silva's day went from bad to worse when he was shown a second yellow for an ill-advised lunge on Okay Yokuslu. His rash action cost Chelsea a game they were clearly dominating and provided West Brom the impetus to come out of their shells.
Interestingly, Thiago Silva's Chelsea debut against West Brom also featured a direct error from the Brazilian, which led to Callum Robinson's goal.
#2 Callum Robinson continues fine run of form against Chelsea
Callum Robinson has scored five goals in his Premier League career, and rather remarkably, all five have come against Chelsea.
He halved the deficit and forced a late own goal in a 2-2 draw at Stamford Bridge while still with Sheffield United last season. Ferguson then followed that up with a 25-minute brace in a 3-3 draw for West Brom in September.
Game-time has been limited for the 26-year-old since then, and he was only on the field due to Branislav Ivanovic's injury. However, Robinson put Chelsea to the sword once again with another brace.