The Baggies took a massive step towards automatic promotion after beating their nearest challengers with a dramatic winner in injury time at the Stadium Of Light.
The Black Cats desperately needed victory to close the nine-point gap but substitute Jason Koumas dashed their hopes when beating Mart Poom from close range.
Albion were rarely in the match as an attacking force, but such was their organisation at the back, Sunderland rarely threatened goalkeeper Russell Hoult.
Both sides lost a key defender through injury in the opening half-hour. Sunderland captain George McCartney was stretchered off after just six minutes with an injury that needed six stitches and Albion's Darren Moore limped off in the 29th minute.
The Black Cats enjoyed most of the possession, but were considerably frustrated by their opponents' negative tactics of pulling every player behind the ball and denying them both time and space.
John Oster and Julio Arca wasted a rare chance to test Hoult with a complete mix up from a free kick just outside the penalty area and the hosts then had strong appeals for a penalty turned down when Gary Breen's centre from the byline was forced behind.
Albion however, should have grabbed the lead in the 37th minute.
Scott Dobie raced clear with the home side appealing in vain for offside but with only Poom to beat, the Scottish international fired high and wide.
Dobie was nearly in again in the next minute. Joachim Bjorklund headed the ball behind for a corner and from Koumas' inswinging corner Poom made a frantic stop at the near post.
The introduction of Kevin Kyle in the 55th minute gave Sunderland a new dimension, the substitute immediately in the thick of the action with his strength and aerial power causing endless distraction Kyle was only inches off target from an Arca corner and then chested down a clever lob from Arca only to shoot over the bar.
Phil Babb almost won it for Sunderland in the last minute but his firm header from Tommy Smith's cross was straight at Hoult and to the dismay of the home fans, Albion snatched victory through Koumas.