The Baggies had moved seven points clear at the summit with a 2-0 victory over Preston North End 24 hours earlier, but Leeds cut the deficit by completing a league double over Middlesbrough.
Fulham moved back up to third after leaving it late to see off Swansea City, while Steven Fletcher struck deep into stoppage time as Sheffield Wednesday triumphed 1-0 over Charlton Athletic.
Millwall missed the chance to move closer to the play-off places, held 0-0 at home by Birmingham City. There were also no goals at Ewood Park, as Blackburn Rovers drew with Stoke City.
KLICH-ING INTO GEAR FOR LEEDS
Marcelo Bielsa's side triumphed 1-0 for a third successive league outing, Mateusz Klich scoring the only goal in first-half stoppage time at the Riverside.
The host managed just two shots on target as it drew a blank yet again – Middlesbrough has not found the net since a 2-2 draw at Wigan on 11 February.
Fulham is five points behind Leeds in the standings; Aleksandar Mitrovic missed an 89th-minute penalty but made amends before the final whistle, grabbing the winner in added time at Craven Cottage.
BATTLE TO BEAT THE DROP HEATING UP
After Luton Town upset Brentford, Barnsley found itself propping up the rest prior to kick-off at Hull City.
However, its recent revival continued with a 1-0 victory, Cauley Woodrow the scorer for the visitor in the 42nd minute.
Wigan still occupies the third and final relegation berth but now only trails Middlesbrough on goal difference after a resounding 3-0 result away at Reading.
Kieffer Moore broke the deadlock with an audacious back-heeled finish, with Jamal Lowe and Michael Jacobs – who struck in the ninth minute of added time – also scoring for the Latics.