Manchester United's Champions League hopes suffered a setback as Brighton scored a dramatic 99th-minute penalty winner to keep their own European ambitions on track.
Alexis Mac Allister converted deep into injury time to settle a Premier League thriller at Amex Stadium.
United - in the fourth and final Champions League qualification spot - stay four points ahead of fifth-placed Liverpool, but now have only one game in hand.
And Brighton deserved this victory for a relentless approach that finally gave them the breakthrough seconds from the end after Luke Shaw handled, the penalty awarded after a video assistant referee review.
Brighton, in sixth, are now only four points adrift of Liverpool, who have played two games more, and their celebrations at the final whistle contrasted sharply with the desolation of their defeat by United on penalties in the FA Cup semi-final at Wembley less than two weeks ago.
Both sides had chances in a superb game, with Antony shooting wide for United in the opening seconds while both Marcus Rashford and Antony Martial forced Brighton keeper Jason Steele into important saves.
Brighton, fresh from the 6-0 thrashing of Wolves, played with the verve that has become their trademark. Kaoru Mitoma shot straight at David de Gea when clean through, and missed another good chance after the break, while Danny Welbeck was also off target with another presentable opening.
The Seagulls turned up the pressure in stoppage time, De Gea saving superbly from Mac Allister and Pervis Etupinan before they finally made the breakthrough.
Brighton's thrilling attacking style under manager Roberto de Zerbi has got their fans dreaming of European football next season and the scenes of celebration at the final whistle reflected the belief and optimism surging through the club.
On this evidence, if Brighton do reach Europe, they will be a wonderfully entertaining addition.
They showed they have recovered from losing to United at Wembley by refusing to take a backward step, hunting down their tiring opponents in the game's later stages.
Brighton play with a pace and intensity that is pure theatre at times and in the end it just proved too much for United, although this game was settled on the fine margin of that late penalty.
Argentina World Cup winner Mac Allister ignored the noise and pressure to beat De Gea convincingly and send the Brighton fans wild. How they enjoyed De Zerbi leading his triumphant players on a lap of honour at the final whistle.
For all Brighton's excellence, they still relied heavily on goalkeeper Steele, who was outstanding when called upon.
Brighton have struggling Everton at home next and in this mood and form, will feel capable of beating anyone.
Manchester United are still in a good position to qualify for the Champions League by finishing in the top four but there is no doubt an indifferent away record is putting them under pressure.
This defeat was their seventh in 17 games, winning seven and drawing three, and for all they made Steele work, the traffic flowed nearly all one way towards their goal as the clock ticked down.
Shaw looked a very guilty man as Brighton manager De Zerbi and his entire backroom staff demanded a penalty in the closing seconds, the inevitable confirmed after VAR invited referee Andre Marriner to visit the screen.
United had their chances but found Steele in top form, their problems coming as they appeared to run out of steam in the second half.
Martial missed one of those opportunities but otherwise gave a lifeless, lacklustre performance and it was no surprise when he was eventually substituted.
United now travel to West Ham United on Sunday and will know their margin for error has been reduced by this defeat.
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