Chelsea were brought back to earth by an inspired West Brom side that took them to school over the course of ninety minutes as Thomas Tuchel's side suffered a major blow in their quest for a top-four finish in this season's Premier League.
Following an unconvincing start, the hosts took the lead after Christian Pulisic pounced on the rebound after Marcos Alonso's free kick struck the post. However, the game took a turn almost immediately as Thiago Silva saw a second yellow card in the space of minutes and was controversially sent off.
The Baggies levelled proceedings in first-half stoppage time as some poor defending from the west Londonders saw Matheus Pereira clip a Sam Johnstone ball over Edouard Mendy.
Sam Allardyce's side took the lead in their next attack after a brilliant bit of play from Pereira, who showed great composure to slot the ball past Mendy to give the West Midlands outfit the lead heading into the break.
West Brom made it three just over the hour mark as Callum Robinson rounded off a silky counter attack with a thumping first-time finish from the edge of the box past Mendy to give the visitors some breathing space.
It soon got better for Albion after Diagne took advantage of some more shaky defending from Chelsea and found the back of the net with an effort which should've been kept out by Mendy.
Mason Mount, who came on for Christian Pulisic at half-time, pulled on back for his side after netting into an open goal after some selfless play from Timo Werner.
However, the Baggies had the last laugh as they tore apart Chelsea's defence on one final occasion in the dying minutes as Robinson bagged his second after some sumptuous play on the break by the visitors.
This was the first defeat that the Blues have suffered since Tuchel was appointed in January, and it was the first time the German boss conceded a goal at Stamford Bridge.
Edouard Mendy - 4.5/10
The Senegalese's positioning could be questioned for the visitors' first and fourth goals. Though he got wrong-footed for the second, on the cusp of half-time, there was very little he could've done for the Baggies' third and fifth. A disappointing outing for the 29-year-old.
César Azpilicueta - 4.5/10
He failed to cope with the threat of Diagne and Pereira down the right flank and was not at his solid, usual best as the Baggies ran riot at the Stamford Bridge, where Chelsea hadn't conceded a single goal under Tuchel prior to the defeat.
Thiago Silva [Red Card - 29] - 3.5/10
The Brazilian was sent off on the half-hour mark after two reckless challenges, which came back to haunt his side as they were outclassed by Albion in the final third.
Kurt Zouma - 3/10
Had a terrible day at the office. Some of the Frenchman's defending was shambolic as he was majorly at fault for the Baggies' first and fifth. He made errors throughout the game and failed to put one away on the other end following a corner situation.
Reece James - 4.5/10
Was ineffective moving forward for a majority of the clash and was often second-best when defending on the right flank as he failed to provide cover for Azpilicueta, who was exploited by the opposition attack on one occasion too many.
Jorginho  - 3/10
Made the mistake that led to Thiago Silva's first yellow. Had a decent first-half otherwise but was absent for much of the second-half. Hardly contributed anything going forward and failed to protect the backline.
Mateo Kovačić - 5/10
He struggled in the midfield alongside Jorginho as his usual passing accuracy was way off against the Baggies. Will need to dust himself down ahead of the clash against Porto next week.
Marcos Alonso - 5/10
He was often his side's most potent attacking outlet as he caused havoc down the left flank. His brilliantly weighted free-kick led to the opener but the defensive side of his game was not nearly up to the mark.
Hakim Ziyech  - 4.5/10
He was unlucky to be sacrificed just after Silva was sent off but had a fairly decent showing before he was subbed.
Christian Pulisic  - 6/10
Gave the hosts the advantage after latching onto the loose ball after Alonso's free-kick hit the post and was subbed off at half-time as Tuchel brought on Mount to inject some creativity into the side.
Timo Werner - 4.5/10
Aside from setting up Mount in the latter stages of the game, he often lacked end product up top but made a few promising strides and passes in the first-half.
Andreas Christensen 
Mason Mount 
Kai Havertz