Manchester City took advantage of Liverpool's late kick-off at Chelsea on Saturday to move top of the Premier League as Manchester United slumped to a third defeat in seven league games, 3-1 at West Ham.
United now trail their city rivals by nine points in 10th and could trail Liverpool by 11 should Jurgen Klopp's men continue their 100 percent record at Stamford Bridge.
City showed the gulf in class between both sides of Manchester as Brighton, who beat United earlier this season, were comfortably seen off at the Etihad 2-0 thanks to goals either side of half-time from Raheem Sterling and Sergio Aguero.
United's worst league start in 29 years means they are also five points adrift of Tottenham and Arsenal in the race for a place in the top four as they were 2-0 victors over Huddersfield and Watford respectively.
- Miserable week -
Defeat at the London Stadium compounded a miserable week for United manager Jose Mourinho having also been dumped out of the League Cup by Derby on penalties and held at home by Wolves last weekend.
Mourinho's switch to a three-man defence backfired as Felipe Anderson and Victor Lindelof's own goal gave West Ham 2-0 half-time lead.
"After the result on Tuesday the team obviously needs positive things, positive feelings and to start losing after five minutes is not that positive start that you need," said Mourinho.
Paul Pogba was also under the spotlight after being stripped of the vice-captaincy by Mourinho in midweek and a frosty training ground exchange between the pair on Wednesday.
However, the French World Cup winner failed to make an impact before he was substituted 20 minutes from time.
Marcus Rashford came off the bench to briefly give United hope by pulling a goal back, but just three minutes later Marko Arnautovic exposed more poor United defending to seal West Ham's first home league win of the season.
- City slickers -
City face a big trip to Hoffenheim in the Champions League in midweek having lost at home to Lyon to open their campaign, but they have suffered no such domestic struggles in defence of their title.
Leroy Sane's pace again made a big impact as the German's low cross was tapped home by Sterling at the back post for his fourth goal of the season.
City could have had many more goals to show for their dominance, but had to wait until 25 minutes from time when Aguero slotted home after a neat one-two with Sterling.
Tottenham moved into fourth as Harry Kane scored twice in nine first-half minutes to easily see off struggling Huddersfield.
However, Spurs suffered a double injury blow days ahead of hosting Barcelona in a huge Champions League clash at Wembley on Wednesday as both Jan Vertonghen and Mousa Dembele had to be replaced at half-time.
Arsenal moved level with their north London neighbours on 12 points with a seventh straight win in all competitions as a Craig Cathcart own goal and Mesut Ozil's finish earned all three points in the final 10 minutes at the Emirates.
Everton ended a four-game winless run to beat Fulham 3-0 as Gylfi Sigurdsson made amends for missing an earlier penalty to score twice either side of Cenk Tosun's first goal of the season.
Wolves' impressive return to the top flight continued thanks to late goals from Ivan Cavaleiro and Jonny Castro to earn a 2-0 win over Southampton.
Newcastle remain winless and in the bottom three after a 2-0 defeat at home to Leicester with two of England's World Cup squad Jamie Vardy and Harry Maguire on target.
