Leeds thrashed Swansea 4-0 to move into second place in the Championship after Southampton's 25-game unbeaten run came to an end with a 3-1 defeat at Bristol City on Tuesday.
Leaders Leicester moved 12 points clear at the top of the table thanks to a 2-0 win over Sheffield Wednesday.
Southampton had not tasted defeat in any competition since September 23.
Russell Martin's men finally came undone at Ashton Gate as second-half goals from Sam Bell, Rob Dickie and Harry Cornick boosted the Robins chances of making the playoffs.
Leeds took full advantage to continue their brilliant form with a seventh consecutive league win.
Early goals from Crysencio Summerville and Joel Piroe on his return to Swansea put the visitors in command.
Italy international Wilfried Gnonto added two more either side of halftime to take his tally to four goals in as many games.
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