Manchester United striker Robin van Persie's soaring first-half header secured a 1-0 win over his old club Arsenal that pricked the visitors' bubble and thrust the champions back into the Premier League title race.
The Old Trafford defeat checked Arsenal's breakaway and left them with 25 points from 11 matches, two clear of Liverpool who thrashed Fulham 4-0 yesterday, with United moving up to fifth place on 20.
Manchester City suffered a stunning 1-0 setback at struggling Sunderland, who had taken just four points from their 10 previous games, and dropped to eighth on 19 points.
Ambitious Tottenham Hotspur also hit an unexpected roadblock in losing 1-0 to Newcastle United at White Hart Lane, with visiting goalkeeper Tim Krul making a stunning 14 saves to keep a clean sheet.
Arsenal's Old Trafford trip was a litmus test for both sides, with the North Londoners arriving as the form team eager to end years of hurt at United and charge 11 points clear of old rivals under new management.
Alex Ferguson may have been watching from the directors' box rather than shouting on the bench but David Moyes' side ensured the outcome remained depressingly familiar for the Gunners, who have now won only one of their last 10 league games against United.
"This was a big result for us today. Arsenal have been in terrific form this season," said Moyes after his biggest success so far at United.
AWAY DEFEAT
It was Arsenal's first away defeat since March after a run of 15 games unbeaten in all competitions while United showed they were picking up pace after a shaky start to the new era under Moyes.
Former Arsenal favourite Van Persie bagged his seventh league goal of the season when the Dutchman rose to deftly head home a Wayne Rooney corner at the far post in the 27th minute.
He celebrated the goal with abandon, sparing the visiting fans no agonies by racing away in delight and pumping the air with a clenched fist.
Arsenal failed to live up to expectations, creating few chances on a strangely muted evening for their record signing, Germany midfielder Mesut Ozil, and could even have found themselves 2-0 behind after an hour when Rooney shot just wide.
The visitors were without their defensive rock Per Mertesacker, sidelined before the start with a stomach virus that also affected three other Arsenal players.
United had to replace Nemanja Vidic at halftime when the central defender suffered a head injury.
"He's gone to hospital," said Moyes. "It's a terrible clash, it's an accident but hopefully he'll be OK."
SUNDERLAND SHOCK
On a day of surprises elsewhere, Swansea City fought back from 2-0 down at home to Stoke to go 3-2 ahead but drew 3-3 after Charlie Adam's stoppage time penalty for the visitors.
Ivorian striker Wilfried Bony scored twice in the second half for Swansea.
Gus Poyet's struggling Sunderland pulled off an electrifying 1-0 win over big-spending Manchester City at the Stadium of Light to give their supporters hope of better times ahead.
Second from bottom, Sunderland chalked up a fourth successive 1-0 home win over City thanks to a 21st-minute goal slotted in by Phil Bardsley from the left side of the box.
With Uruguayan manager Poyet pacing around on the touchline as Sunderland faced a late City barrage, the Black Cats held on for their first clean sheet in 15 matches and a stunning win over a side who hammered Norwich City 7-0 last weekend.
"They were outstanding," beamed Poyet, who had defender Wes Brown making his first Premier League start in nearly two years, after his third win in as many home games since replacing sacked Italian Paulo Di Canio.
"We tried to plan the game in three days and tried to convince them it was possible, especially to keep a clean sheet against a top, top team."