Misfiring Arsenal spurned the opportunity to reclaim first place in the Premier League as they lost 2-0 to spirited fellow Londoners West Ham United at the Emirates on Thursday.
West Ham only had three efforts on target but Tomas Soucek gave them a halftime lead before former Arsenal defender Konstantinos Mavropanos extended the advantage after the break.
Arsenal had more than twice as many chances but failed to take any as the visitors defended gamely.
Second-placed Arsenal remain on 40 points, two behind Liverpool who beat Burnley 2-0 on Tuesday.
"This is a very disappointing night for us," Arsenal manager Mikel Arteta told reporters. "We have to put the ball in (the net) and win it (the game)."
West Ham took the lead against the run of play after 13 minutes when Soucek netted after a pull-back from Jarrod Bowen who looked as if he just managed to keep the ball in play.
"It's difficult because the ball was in the air," Bowen told Amazon Prime. "It was hard to tell. It would have been easier to tell if it had been on the ground."
After that it was an Arsenal blitz with Bukayo Saka hitting the post and Gabriel Jesus missing two good chances before Mavropanos scored his first league goal for the Hammers with a thumping header from a James Ward-Prowse corner in the 55th.
"I think the best moment to score my first goal for West Ham and in the Premier League," said Mavropanos. "We work a lot on the set-pieces, so it was great. It is an important win for everyone and for our boss. We are really happy."
West Ham could have made it three from three chances but Said Benrahma had a penalty saved by David Raya in added time after a foul by former West Ham captain Declan Rice.
The defeat raises familiar questions about Arsenal's title credentials. They led the league this time last year only to falter in the run in and be overtaken by Manchester City.
It may have been one of those nights when things just did not happen in front of goal but finishing remains an issue.
As well as striking the post, Saka also had a near point-blank header stopped by Alphonse Areola, who made a string of fine saves in the second half too.
Jesus should have a scored when Arsenal were still one goal behind but headed over an open goal while fellow Brazilian Gabriel Martinelli also missed good opportunities.
The win leaves West Ham in sixth place on 33 points and marked a landmark night for their manager David Moyes who had never won at Arsenal in the league in 22 attempts.
Late rally fails to save Tottenham in 4-2 loss at Brighton
Tottenham Hotspur's top-four ambitions suffered a blow as they lost 4-2 at Brighton & Hove Albion in the Premier League on Thursday with only a late rally saving them from their heaviest defeat since Ange Postecoglou became manager this season.
Victory would have put injury-hit Spurs above Manchester City into fourth spot but they were mauled on the south coast as a Brighton side also missing key players ran riot.
A powerful early strike by Josh Hinshelwood and Joao Pedro's penalty gave the hosts a deserved 2-0 lead at halftime as Tottenham struggled to deal with Brighton's intensity.
The visitors responded in the second half with Richarlison having a goal disallowed but when halftime substitute Pervis Estupinan sent a thunderbolt past Spurs keeper Guglielmo Vicario and Joao Pedro netted a second penalty the game seemed over.
However, Tottenham's late substitute Alejo Veliz fired home from close range in the 75th minute and Ben Davies scored with a header making a miraculous comeback suddenly look possible.
Midfielder Pierre-Emile Hojbjerg hit the post for Spurs in stoppage time as Brighton looked nervy but they held on to move into eighth place in the table on 30 points while Tottenham remain fifth on 36, a point off the top four.
The final scoreline hardly did justice to a chaotic match in which Tottenham were second-best for long periods but showed some spirit to emerge with their pride intact.
However Brighton, without the likes of winger Kaoru Mitoma, Tariq Lamptey, Solly March and Julio Enciso, were worthy winners as they snapped a three-match league sequence without a win.
"It is a very disappointing performance. This is not how we've approached the season," Spurs captain Son Heung-min said.
"This performance was nowhere near the level we want to play at. But now is not the time to point fingers at each other because the games are coming fast."
Tottenham host in-form Bournemouth on Sunday when they will be without Kulusevski who will be suspended after picking up another booking, while Brighton visit West Ham United on Jan. 2.
Tottenham's defensive problems, made worse by Argentine centre back Cristian Romero having been added to their long injury list, were exposed by Brighton as the hosts dominated the first half with some slick, high-intensity football.
Vicario had already made two fine saves before Hinshelwood was played in by Joao Pedro on the edge of the area and thumped an unstoppable shot high into the net.
Brighton almost doubled their lead when Jan Paul van Hecke's header hit the post and as the ball rebounded towards Danny Welbeck the striker was blatantly pulled back by Dejan Kulusevski with a penalty being awarded after a VAR check.
Joao Pedro rolled the 23rd-minute spot kick past Vicario to put Brighton in command although Richarlison nearly responded just before halftime with a shot that kissed the post.
Spurs looked brighter after the break and wasted several chances before Brazilian Richarlison netted, only to have his joy cut short as VAR showed he was offside.
Estupinan made it 3-0 with a sensational strike from outside the area before Tottenham substitute Giovani Lo Celso conceded a penalty that Joao Pedro dispatched.
A raft of substitutions revived Spurs with Bryan Gil and Veliz adding some energy to the visitors and the latter celebrated wildly as he slotted his first goal for the club.
Davies headed in from close range and Tottenham then had numerous chances to try to grab an unlikely point although they can have no real complaints about leaving empty handed.
