(CNN) -- English Premier League champions Chelsea suffered a humiliating 3-0 home defeat to Sunderland to see their lead at the top cut to two points as Arsenal won 2-1 at Everton on Sunday.
Carlo Ancelotti's side were without the injured John Terry and Frank Lampard, as well as the suspended Michael Essien, but few could have predicted such a stunning upset against a team who were thrashed 5-1 themselves at Newcastle a fortnight ago.
A virtuoso solo goal by defender Nedum Onuoha put the visitors ahead just before halftime.
Asamoah Gyan added the second with man of the match Danny Welbeck grabbing a late third as he took advantage of a misplaced backpass from Ashley Cole.
It completed a difficult week for the Blues, who were criticized for their abrupt sacking of assistant manager Ray Wilkins, before learning Lampard would be ruled out until December.
"Everything went wrong, we didn't play well or how we wanted," Ancelotti told Sky Sports.
Asked about the controversial departure of Wilkins, the Italian retorted: "It was not a reason."
Sunderland had chances to score before Onuoha's opener with Petr Cech denying Welbeck as he burst clear.
Chelsea's Branislav Ivanovic was then fortunate to escape a red card for fouling Welbeck before Onuoha waltzed past three defenders before scoring with a clever low shot.
Gyan took advantage of a perfect pass from new England squad man Jordan Henderson to double the lead before Welbeck got reward for his efforts to complete the rout.
Earlier, Arsenal moved above Manchester United into second place with a fine win at Goodison Park.
Arsene Wenger's team took the lead through French defender Bacary Sagna's first half goal and Cesc Fabregas grabbed the second soon after halftime.
Tim Cahill scored a consolation for Everton in the final minute but Arsenal were well worth the three points to boost their growing title challenge.
United had to come back from two down at Aston Villa to salvage a 2-2 draw on Saturday.