There was a huge twist in the title race as David Moyes's West Ham shock Chelsea in the Londen derby.
Chelsea took the lead twice, but West Ham responded both times with goals from Manuel Lanzini and Jarrod Bowen.
Arthur Masuaku scored the winner for West Ham in the 87th minute to sink Chelsea.
West Ham solidify their position in the top four while Chelsea lose their lead at the top with Liverpool and Manchester City picking up wins.
Romelu Lukaku continues to sit on the bench for Chelsea as Kai Havertz is deployed as the centre forward. Thiago Silva, Hakim Ziyech and Jorginho return to the starting line-up after missing the midweek game at Watford.
Chelsea XI: Mendy, Christensen, Silva, Rudiger, James, Loftus-Cheek, Jorginho, Alonso, Mount, Havertz, Ziyech
Subs: Kepa, Azpilicueta, Sarr, Saul, Barkley, Hudson-Odoi, Werner, Pulisic, Lukaku
There was a formation change from West Ham as David Moyes opted for a back three in order to contain Chelsea. Issa Diop and Lanzini made their first starts of the season replacing Benrahma and Pablo Fornals.
West Ham United: Fabiański, Coufal, Dawson, Diop, Zouma, Johnson, Rice (c), Souček, Lanzini, Bowen, Antonio.
Subs: Areola (GK), Ashby, Masuaku, Noble, Král, Benhrama, Vlašić, Yarmolenko, Fornals.
FIRST HALF: Chelsea take the lead twice
Chelsea dominated possession in the first half as West Ham were happy to sit back and defend. The first chance fell for West Ham though as Bowen latched onto a poor ball from Thiago Silva and fired in a shot that went wide. Thiago Silva made another error as his header was intercepted by Lanzini and Antonio's cross was headed wide by Thomas Soucek.
Chelsea took a deserved lead at the half-hour mark as Thiago Silva scored from a free header. The hammers could have equalized straight away through Vladimir Coufal but his shot was cleared off the line by Thiago Silva. Chelsea gifted the equalizer with shambolic passing at the back as Jorginho played a dangerous back pass to Mendy. Bowen pressurized the goalkeeper who could have cleared it but lost the ball and fouled Bowen to give away a penalty. Lanzini made no mistake from the spot to score his second goal of the season.
Chelsea took the lead for the 2nd time at the stroke of half-time as Ziyech's looping pass was volleyed home by Mason Mount.
SECOND HALF: Bowen and Masuaku win it for the Hammers
West Ham started the second half brightly and had some good possession. They got the equalizer 10 minutes into the second half through Jarrod Bowen. West ham should have taken the lead 20 minutes later but Bowen couldn't meet Antonio's cross to put it in an open net.
The Hammers took the lead with 3 minutes to go through a freak goal from Masuaku. He scored from an almost impossible angle and Mendy couldn't sort his feet quickly and palmed it into his own net.
David Moyes was understandably delighted with his team's performance but suggested there is room for improvement, especially in the first half.
"Technically we can play much better than we did in the first half, we didn't play well enough in the first half today. I thought the second half we improved greatly and that gave us a real chance but I was patching up the team at different times where we picked up a few injuries,
" he said in the post-match interview