Brighton edged three points clear of third-bottom Fulham with a game in hand after a frenetic south-coast derby at St Mary’s, winning 2-1.
It was a first win in eight games in all competitions for Graham Potter’s side since they beat Tottenham in January.
The Seagulls are one point above fourth-bottom Newcastle, who travel to Brighton for a crucial clash in the relegation battle next weekend.
Brighton took the lead through Lewis Dunk in the 16th minute.
Fraser Forster made a fine save to keep out Neal Maupay’s shot but Dunk’s towering header from the resulting corner flashed past the Saints keeper.
Southampton drew level in the 27th minute when Dunk could only help Ryan Bertrand’s header into the path of Che Adams, who volleyed home at the far post.
But Brighton went back in front in the 56th minute as Leandro Trossard produced a fine finish from Danny Welbeck’s lay-off.
Later on Sunday, Tottenham, unbeaten in the last five north London derbies dating back to 2018, will aim to bolster their top-four bid by recording a third successive victory against bitter rivals Arsenal.
Jose Mourinho’s side have won their last five games in all competitions and can close to within three points of fourth-placed Chelsea with a game in hand if they win at the Emirates stadium.
West Ham are also in the race to qualify for the Champions League via a top-four finish.
David Moyes’ side can move level on points with Chelsea with a game in hand if they beat Manchester United at Old Trafford.
The Hammers have won eight of their last 11 league matches, while United need the points to cement their own top-four ambitions.
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United dropped to third after Leicester’s victory, but Ole Gunnar Solskjaer’s men would go back above them with three points against West Ham.
Former United boss Moyes, sacked by the Old Trafford outfit in 2014 after less than a season in charge, will be returning to his old club for the second time this term following their FA Cup defeat last month.