Arsenal missed out on the chance to return to the Premier League summit on Thursday, losing 2-0 to West Ham as Tottenham went down in a six-goal thriller at Brighton.
Mikel Arteta's Gunners were top of the tree at Christmas for a second successive year but trail leaders Liverpool by two points at the halfway point of the season.
Last year's runners-up dominated possession at the Emirates but lacked a cutting edge against the disciplined visitors, for whom Tomas Soucek and former Arsenal player Konstantinos Mavropanos scored the goals.
Soucek opened the scoring from close range in the 13th minute following a pass from Jarrod Bowen, with the goal confirmed after a lengthy VAR check looking into whether the ball had gone out of play.
Arsenal surged forward in relentless waves and West Ham goalkeeper Alphonse Areola made a fine reaction save from a Bukayo Saka header on the half-hour.
The England winger hit the post from a tight angle as the clock ticked down towards half-time but Arsenal trailed at the break despite enjoying nearly three-quarters of possession.
The Gunners came out determined to find a response but instead they found themselves 2-0 down when Mavropanos's bullet header cannoned in off the crossbar from James Ward-Prowse's corner.
Gabriel Jesus failed to beat Areola with a header when well-placed and shortly afterwards missed another glorious opportunity with his head at the back post.
Arsenal continued to bombard the visitors' goal but could not find a breakthrough and it could have been worse had Said Benrahma managed to convert a late penalty.
Defeat leaves the Gunners in second place on 40 points, one clear of Aston Villa and three ahead of Manchester City, who have a game in hand.
Arteta said his team deserved more for their dominance.
"We want to be at the top and today we are very, very disappointed," he told the BBC.
"We had so much dominance in the game. We are in a really good position. Today is a disappointing night. We have to pick ourselves up for a big game against Fulham."
He added: "The players put an enormous effort in. It's a difficult dressing room right now but we have another game in 72 hours. This is football, we have to improve in certain areas."
West Ham manager David Moyes celebrated his team's rise to sixth in the table.
"We are not one of the top teams," he told Amazon Prime. "We are trying to get up and grow the team. For West Ham to be in Europe (they topped their Europa League group) is a great period."
Tottenham, this season's early pacesetters, remain in fifth spot after losing 4-2 despite threatening an astonishing comeback from 4-0 down against Roberto De Zerbi's Brighton.
Jack Hinshelwood finished with brutal power to put the home side ahead following a weaving run from Joao Pedro along the edge of the penalty area.
Brighton doubled their lead from the penalty spot in the 23rd minute through Pedro, who took full advantage after Dejan Kulusevski was penalised for a pull on Welbeck.
Brazil forward Richarlison twice had the ball in the net after the break but both times he was ruled offside.
Pervis Estupinan stretched the lead with a thunderbolt from 30 yards in the 63rd minute and Pedro scored a second penalty 12 minutes later.
Alejo Veliz grabbed what appeared to be a consolation goal in the 81st minute but it galvanised the visitors.
Ben Davies headed home Pedro Porro's deep cross as Spurs threatened to cut through every time they poured forward, with Pierre-Emile Hojbjerg hitting the post during nine frantic minutes of stoppage time.
But Brighton withstood the intense pressure to take all three points.
"I am really lucky to be this coach of this team with these players," De Zerbi told the BBC. "I am very pleased with the passion they have, they have everything.
"You can't forget we are Brighton. Sometimes they forget we are Brighton. We are not Manchester United, Manchester City, Newcastle or Arsenal, we are still Brighton."
