Joao Pedro netted a late winner to halt Aston Villa's charge to qualify for the Champions League as Brighton deservedly claimed a 1-0 victory at the Amex Stadium.
Villa had the chance to move 10 points clear of fifth-placed Tottenham with a win, knowing a defeat for their top-four rivals to Liverpool on Sunday afternoon, live on Sky Sports, would see them secure fourth.
But Unai Emery's side looked jaded after their midweek defeat to Olympiakos with the out-of-form hosts denied the opening goal during the second half by VAR when Pascal Gross was ruled offside.
Villa's John McGinn then found the net but he too was offside before Brighton deservedly claimed the points, Pedro heading in the rebound from his penalty which the brilliant Robin Olsen had saved.
Villa remain in fourth, seven points above Spurs, who have two games in hand.
How Brighton rocked Villa
Brighton winger Simon Adingra was the best player throughout and he forced Olsen, who filled in for the injured Emi Martinez superbly, into an early save.
Villa struggled to find any rhythm which was hindered further when the in-form Morgan Rogers was forced off in just the 24th minute with a muscle injury.
Ollie Watkins showed his quality with a nice touch to get away from Adam Webster yet Villa saw very little of the Brighton goal with all the action coming up the other end.
The hosts had two chances in added time with Adingra unable to steer a cross past Olsen before the Villa goalkeeper made a crucial save from Gross at close range.
Olsen continued his fine form in the second half when Adingra played Joao Pedro through on goal and the striker was denied by the towering goalkeeper.
Brighton thought they had finally taken the lead when Gross turned in from Igor's left-wing cross but after a lengthy VAR check, the goal was ruled out.
Villa then saw McGinn's strike flagged offside when he had tucked home after the ball ricocheted to him inside the box while Brighton striker Danny Welbeck saw his header drop wide.
Brighton pushed for a winner and got their reward when Adingra got the wrong side of Ezri Konsa and went down in the box. Olsen dived to his right and kept out Pedro's penalty, but the ball sat up nicely for the Brighton forward to head in.