The two Manchester clubs slumped to shock defeats and Arsenal moved two points clear with a win at Swansea on Saturday, extending the unpredictable start to the English Premier League.
Five months after capturing a 20th English title, Manchester United is languishing in 12th place in the 20-team standings after losing 2-1 at home to West Bromwich Albion in the latest setback to hit David Moyes' troubled first season in charge.
Amid its worst six-game start to a league season since 1989, United slipped eight points behind Arsenal after the London club beat Swansea 2-1 for an eighth straight away victory in the league.
The only solace for United on an action-packed day was neighbour Manchester City enduring similar misery by sinking 3-2 at Aston Villa.
With the Manchester giants showing fluctuating form, their rivals from London are taking full advantage. Tottenham is two points behind in second after drawing 1-1 at home to third-place Chelsea in an entertaining match that saw Fernando Torres sent off for the visitors.
"I'm concerned after today's result, but only because we didn't play well," said Moyes, whose team took another blow six days after losing 4-1 to City in the derby. "But there are a lot of games to go and I'll try to put it right."
'I'm concerned after today's result, but only because we didn't play well. But there are a lot of games to go and I'll try to put it right.' - Man U manager David Moyes
Even promoted teams Cardiff and Hull are above United after six games. Cardiff won its first away match of the season, 2-1 at Fulham, and Hull edged West Ham 1-0.
But another newcomer, Crystal Palace, lost 2-0 at Southampton, which is adding to the surprise factor this season by sitting in fourth place.
Sunderland is lodged to the bottom on one point ahead of Sunday's game against Liverpool. Unbeaten Everton hosts Newcastle on Monday.
Arsenal midfielder Aaron Ramsey maintained his stunning start to the season by setting up Serge Gnabry's opener in the 58th then scoring himself four minutes later at Liberty Stadium. Ben Davies cut the deficit in the 81st but Arsenal held on and is confounding expectations.
"We have a lot of quality," said Ramsey, who has scored eight goals in nine matches this season. "We've had a lot of injuries recently but we've managed to dig in and put a load of wins together and hopefully now we get players back and we're in with a shout [of winning the title]."
West Brom fully deserved its first win at United in 35 years, with Saido Berahino clinching victory with a 67th-minute strike from the edge of the box that finished off a flowing team move.
Wayne Rooney's free kick score cancelled out Morgan Amalfitano's stylish solo goal, but a United side missing several regular first-team stars with a Champions League match at Shakhtar Donetsk in mind struggled throughout. Moyes' team hasn't scored from open play since the first day of the season.
City was toasting derby glory last weekend, but was brought to earth by the loss at Villa to drop to fifth.
City twice threw away the lead given by Yaya Toure and Edin Dzeko before Andreas Weimann struck Villa's winner to leave the visitors without an away win this season.
"We played very well and deserved to win, but in five minutes [when Villa scored twice] we threw away everything good we did in the rest of the match," City manager Manuel Pellegrini said. "The way we lost this game was incredible."
Tottenham showed it should be held up as a genuine title contender by holding Chelsea to a draw at White Hart Lane, although it will be disappointed the team couldn't kick on after Gylfi Sigurdsson opened the scoring in the first half.
Chelsea seized the initiative in the second half, with John Terry equalizing with a header from Juan Mata's free kick but hopes of victory receded with around 10 minutes to go when Torres was shown a second yellow card for an aerial collision with Jan Vertonghen.
"I'm sad because the game was good," Mourinho said. "In the first half it was not such a good Chelsea, in the second half Chelsea was not good — Chelsea was very, very good."
Cardiff inflicted more misery on Fulham manager Martin Jol after substitute Jordon Mutch curled home the winner in the second minute of injury time. Steven Caulker's opener for Cardiff in the 12th had been cancelled out in the 45th by substitute Bryan Ruiz.
Robbie Brady won and converted a 12th-minute penalty for Hull while Pablo Osvaldo and Rickie Lambert — with his 200th league goal — were the scorers for Southampton.