Arsenal's Premier League title bid was dealt a blow after they slumped to a surprise 2-0 defeat by in-form West Ham.
Former Arsenal defender Konstantinos Mavropanos' emphatic second-half header added to a controversial early effort from Tomas Soucek as the notably below-par Gunners suffered their first home loss in any competition so far this season at the Emirates Stadium, despite David Raya saving Said Benrahma's penalty at the death.
A third league loss of the campaign meant that Arsenal missed the chance to leapfrog title rivals Liverpool and go back to the top-flight summit.
Instead, Mikel Arteta's side will make the short trip for another London derby at Fulham on New Year's Eve sitting second and still two points adrift of the Reds, who later entertain Newcastle at Anfield on New Year's Day.
For West Ham, a sixth victory from their last eight league games saw them rise above Manchester United - whom they beat 2-0 at the weekend - and into sixth place, only four points adrift of the top four before hosting Brighton on January 2.
It is the first time they have reached the halfway stage of a Premier League campaign with as many as 33 points in the bag as boss David Moyes also celebrated the maiden away win at Arsenal - where West Ham had a disastrous recent record - of his long managerial career, coming with both first-choice centre-backs sidelined in Kurt Zouma and Nayef Aguerd.
However, their joy was tempered by an injury to Lucas Paqueta, who started the game despite a knock suffered in the warm-up but later limped off before half-time.
West Ham's first goal was mired in some controversy as Jarrod Bowen just missed a dangerous Emerson Palmieri cross from the left and Oleksandr Zinchenko's attempted clearance slammed into Gabriel, with Bowen hooking the ball back from the byline before it was drilled home emphatically by Soucek.
The on-field decision was goal and it stood after a lengthy VAR check, with no conclusive proof found that the ball had gone out of play before being turned back by Bowen after multiple replays and several different camera angles.
But there was no doubting West Ham's second goal, a trademark James Ward-Prowse set-piece delivery headed powerfully in off both the crossbar and post by summer signing Mavropanos, who did not celebrate against the club where he spent four years between 2018-22 but was sent out on loan to the Bundesliga for most of that time.
Things almost got even better for West Ham late on, but a last-gasp Benrahma penalty awarded for Declan Rice's clumsy foul on Emerson was kept out by Raya.
At the other end, off-colour Arsenal rarely provided a stern test for the second-choice centre-back pairing of Angelo Ogbonna and Mavropanos, who both struggled notably in the 5-1 Carabao Cup quarter-final thrashing by Liverpool last week but stood up to another tough challenge very well here.
Alphonse Areola was alert to deny the likes of Martin Odegaard and Bukayo Saka, Arsenal's biggest threat on the night who also hammered against the post shortly before half-time.
