Everton sent Chelsea crashing to their third Premier League defeat of the season with striker Steven Naismith scoring a hat-trick in a 3-1 win after coming on as a first-half substitute at Goodison Park.
The result left the champions with four points from their opening five matches - their worst start to a season since 1988.
4 Callum Wilson (Bournemouth)
Riyad Mahrez (Leicester City)
Bafetimbi Gomis (Swansea)
3 Steven Naismith (Everton)
Andre Ayew (Swansea)
Naismith replaced the injured Mo Besic and headed Everton in front in the 17th minute. He doubled Everton's lead five minutes later before Nemanja Matic brought Chelsea back into the game with a superb 25-metre strike.
But Naismith wrapped up the points for the home side with another well-worked goal after 83 minutes.
Elsewhere, Anthony Martial introduced himself to Manchester United fans with a superb goal to complete a 3-1 win over bitter rivals Liverpool and cap a much-improved performance from Louis van Gaal's side.
Martial, a $55.54m signing from Monaco, danced past three defenders in the 86th minute and sidefooted the ball home on his debut to put the finishing touches to a well-deserved victory.
After a dire first half that would have dampened the enthusiasm of even the most die-hard fan, a fired-up United emerged after the break to take the game by the scruff of the neck.
Daley Blind curled them ahead from the edge of the area four minutes after the restart before Ander Herrera slammed a penalty into the roof of the net.
Liverpool hit back with six minutes remaining when Christian Benteke pulled off an acrobatic overhead kick but he was spectacularly upstaged when Martial, a second-half substitute, had the United fans drooling in anticipation of his potential.
|Manchester United sit five points behind leaders Manchester city [EPA]
Meanwhile, Manchester City opened up an 11-point gap over Chelsea when they beat Crystal Palace 1-0 with a stoppage-time goal at Selhurst Park.
City looked set to drop their first points of the season until teenage substitute Kelechi Iheanacho struck late at Selhurst Park.
Manuel Pellegrini's team moved to 15 points from five matches and they top the table from Arsenal, who grabbed their first home win and goals of the season with a 2-0 victory over Stoke City to move up to 10 points.
Walcott broke the deadlock in the 31st minute after running onto a perfect through ball from Mesut Ozil, and Giroud then came on a substitute to put the result beyond doubt in the 85th.
Arsenal 2-0 Stoke City
Crystal Palace 0-1 Manchester City
Everton 3-1 Chelsea
Norwich 3-1 Bournemouth
Watford 1-0 Swansea City
West Brom 0-0 Southampton
Source: Al Jazeera and agencies