Manchester United striker Robin van Persie's first-half header secured a 1-0 win over his old club Arsenal on Sunday that 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 on Saturday, 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 from 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
"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.
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."
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."
Krul was the hero at White Hart Lane in what he said was the game of his life.
"I'm delighted, I'm over the moon... this is a highlight of my career," grinned the Dutchman who was man of the match after making more saves than any Premier League goalkeeper in a match this season.
"These kind of games don't happen very often. Three points away at Tottenham is a fantastic result."
The focus was on both goalkeepers from the start, with Hugo Lloris left out of the Tottenham squad on medical advice after the French international had been knocked unconscious at Everton last week.
His 42-year-old replacement Brad Friedel had to make a couple of saves early on but was powerless to prevent the livewire Loic Remy firing Newcastle in front after 13 minutes.
The France striker picked up a beautifully-measured pass from Yoan Gouffran to dart around the American keeper and stab his seventh league goal of the season.