Two late goals saw Leeds fall to a 2-0 Yorkshire derby defeat at Elland Road, which meant West Brom moved to the summit despite dropping points in a dramatic 2-2 draw at Charlton Athletic.
Brentford and Fulham are third and fourth respectively after wins over Queens Park Rangers and Hull City, with Nottingham Forest dropping to fifth after it was held to a last-gasp 1-1 draw at Reading.
Bristol City, Birmingham City and Barnsley were the other winners, while there were draws when Blackburn Rovers hosted Preston North End, Middlesbrough took on Derby County and Stoke City faced Millwall.
MISFIRING LEEDS SLIPS UP AGAIN
Leeds has only won one of its past seven games in all competitions and now trails West Brom by a point, with Brentford only six points adrift in third.
Bielsa's side looked like it would have to settle for a goalless draw but it got worse when Jacob Murphy scored a goal that goalkeeper Kiko Casilla will not remember fondly with three minutes left.
Atdhe Nuhiu, who had set up the opener, then made the points safe in stoppage-time as Wednesday moved up to sixth.
Patrick Bamford had earlier missed a good chance as well as having a strike disallowed for offside, while Jack Harrison was also guilty of not converting a clear opening.
The Bagies twice led against Charlton at The Valley but had to settle for a draw courtesy of Josh Davison's first career strike and an own goal from keeper Sam Johnstone, who was trying to keep out Tom Lockyer's header.
Albion's goals came from a superb Kenneth Zohore effort and a deflected strike from Hal Robson-Kanu.
The two teams also drew 2-2 in the reverse fixture in October, Charlton coming from behind on that occasion too.
BRENTFORD AND FULHAM KEEP UP PRESSURE
Brentford raced into a three-goal lead after 33 minutes at home to QPR, Ollie Watkins heading in his 18th league goal of the season in a 3-1 derby victory at Griffin Park.
An excellent long-range strike from Ivan Cavaleiro, who joined Fulham permanently from Wolverhampton Wanderers this week, put Scott Parker's men in front against after 29 minutes and that proved to be the only goal of the game.
Forest is a point and a place behind Fulham after a dramatic conclusion to its game at Reading.
Ben Watson appeared to have given Forest an away victory six minutes into stoppage-time but Reading went straight up the other end and Jordan Obita's cross resulted in a costly own goal from Tobias Figueiredo.
Luton Town remains bottom of the table after Gary Gardner's strike earned Birmingham a 2-1 away win. Wigan Athletic lost 2-0 at home to Bristol City and remains in the bottom three along with Barnsley, which won 2-1 at home to Huddersfield Town to move within a point of safety.