Jermain Defoe's eighth goal of the season condemned Premier League champions Leicester to yet another defeat as improving Sunderland won for the third time in four attempts.
Defoe struck with 13 minutes remaining after Robert Huth's own goal had given the home side the lead, and, although substitute Shinji Okazaki pulled one back for the visitors within two minutes, there was no way back.
The Foxes were once again a shadow of the side which romped to the Premier League title last season and the Black Cats took full advantage to win 2-1 in front of a delighted crowd of 39,725 to further boost their survival hopes.