Brighton downed Arsenal who fell to their second successive defeat, putting the race for a top-four spot in the Premier League out of their hands.
Enock Mwepu set up Leandro Trossard for the first and then scored the second to seal the game.
The opener came just before the half-hour mark when Mwepu got behind the space left by makeshift left back Granit Xhaka and from the by-line picked out Trossard who coolly side-footed home into the top right-hand corner.
Midway through the second half, Moises Caicedo dribbled deep in the Arsenal area before clipping a ball back to the edge of the box which Mwepu met on the half-volley with an unerring low finish.
Arsenal were given hope a minute from time when, less than 60 seconds after hitting the bar from a free kick – and Eddie Nketiah smashing the follow up against the woodwork – Martin Odegaard’s effort from 30 yards was deflected off Danny Welbeck and flew over the head of Robert Sanchez.
In-between Arsenal had a goal ruled out by VAR when Gabriel Martinelli headed home from a yard out.
The result leaves the Gunners, who fell to a 3-0 defeat at Crystal Palace on Monday, behind their North London rivals Tottenham.
Brighton, who snapped a run of six defeats and a draw in seven games, rise to 11th spot.
TALKING POINT – SHOULD VAR MAKE EDUCATED GUESSES?
This is what the decision to deny Gabriel Martinelli’s goal was. It must have been. He was close to the goal-line when Gabriel headed the ball from a yard behind at the back post, but clearly Robert Sanchez’s foot was playing him on. Sanchez was also obscuring Marc Cucurella’s body from the television angle from the right of the play. It is likely Cucurella was slightly in front of Martinelli but without an angle to see this, can a decision really be made? It seems amazing at a high-tech stadium there was no angle from both sides of the goal which is surely something which will be in place for venues in the World Cup later this year.
MAN OF THE MATCH – ENOCK MWEPU (BRIGHTON)
It has been a stop-start beginning to the Zambian’s career at the Amex Stadium, but his only run of games before now coincided with Brighton’s good run of form at the turn of the year. After a two-goal assist game against Everton he was out for two months with a hamstring injury and his return to the starting XI certainly gave Brighton a dynamic presence they often lack. Whether on the right flank or in the middle of the park his tough tackling and driving runs forward played a big part in Brighton seizing control of the game. Even before his fantastic game sealer, there was one particular magic period of play where he intercepted a pass, nutmegged Emile Smith Rowe then jumped over a challenge before delivering a perfect square ball to continue a counter-attack.
Arsenal: Ramsdale 6; Cedric 7, White 6, Gabriel 6, Xhaka 5; Lokonga 6, Odegaard 6, Smith Rowe 5; Saka 6, Lacazette 6, Martinelli 6.
Subs: Pepe 6, Nketiah 6.
Brighton: Sanchez 7, Dunk 8, Veltman 7, Cucurella 7; Mwepu 9*, Bissouma 6, Caicedo 7, Mac Allister 6, Gross 6; Welbeck 6, Trossard 7.
Subs: Lamptey 6, Lallana 6, Webster 6.
28′ GOAL FOR BRIGHTON! Mwepu gets to the by-line behind Xhaka and cuts back for Trossard just inside the area and his side-footer nestles in the top right-hand corner.
45+2′ GOAL FOR ARSENAL! Martinelli heads home from close range after Gabriel heads the ball back across the goal and the Brazilian nods home on the goalline. We will check for offside though.
45+6′ GOAL RULED OUT! Martinelli was in front of Gabriel when he headed the ball. It is hard to see he was conclusively behind one of the defenders whose bodies were obscured by Sanchez. He probably was off but certainly can’t have been definitive.
66′ GOAL FOR BRIGHTON! Mwepu with a fantastic low strike from the edge of the box on the half-volley which gave Ramsdale no chance after Caicedo played a one-two with Trossard and clipped a ball back to him.
88′ OFF THE CROSSBAR! TWICE! Odegaard’s shot has Sanchez beaten but comes back off the bar and then Nketiah with the follow up also hits the woodwork.
89′ GOAL FOR ARSENAL! Odegaard, now full of confidence in his shooting ability, shoots from 30 yards and his shot deflects off Welbeck and over Sanchez and into the net.