Brighton stunned a lacklustre Manchester United with three first-half goals to secure a famous victory at the Amex Stadium.
The Seagulls beat United in May to retain their Premier League status, but few expected a repeat on Sunday.
However, Brighton capitalised on a poor start from United to take the lead on 25 minutes, when Glenn Murray flicked home Solly March's low cross.
And they doubled their lead just two minutes later, Shane Duffy taking advantage of the visitors' inability to clear by stroking home his first Premier League goal.
Romelu Lukaku halved the deficit 11 minutes before the break, rising unmarked to nod in Luke Shaw's ball.
But Eric Bailly's rash challenge on Pascal Gross inside the area presented Brighton with the chance to restore their two-goal advantage, which Gross took.
United improved marginally after the break but never really looked like threatening a fightback, though Paul Pogba netted an injury-time penalty after Duffy brought down Marouane Fellaini.
It was a day to forget for United fans, especially after their neighbours City swept aside Huddersfield 6-1.
Mourinho's fears come to fruition
United boss Jose Mourinho warned in the summer that his side faced "a difficult season" if they did not strengthen before the transfer window closed.
The Portuguese had highlighted his side's defence as an area that needed bolstering, but deadline day passed without any new arrivals.
Mourinho's concerns were laid bare here, with his defence caught out on a number of occasions by Brighton's quick movement.
Bailly, in particular, struggled. Aside from giving away the penalty he also miscontrolled a pass in the second half that momentarily put March through before the Ivorian was able to recover.
Bailly and centre-back partner Victor Lindelof - both Mourinho signings - have now been a part of a defence that has conceded four goals in the first two games of the season. It took United nine to let in that many last season.
Fortress Amex as intimidating as ever
Brighton's home form played a huge part in their survival last season, with 29 of their 40 points secured at the Amex Stadium.
Once again, it looks like a trip to south coast will be a difficult one for their Premier League rivals.
United may have been well below par but that should not take away what was a controlled and confident display by the Seagulls against a side that finished second last season.
They took the game to their opponents from the outset, March working the defence with an early cross, and even the loss of influential defender Lewis Dunk did not disrupt them as Anthony Knockaert got in behind before putting his shot just wide.
Brighton did not have a shot after the break, but they did not need to as they changed tack from attack to containment. In the end it was a comfortable afternoon for Chris Hughton's side.
Martial fails to take his chance
Alexis Sanchez was a surprise absentee from United's starting XI, with Mourinho confirming before kick-off that the forward was injured.
That meant Anthony Martial, who has been linked with a move, was handed the chance to impress.
Mourinho was critical of Martial for his performance here in May, and another ineffective display will surely not have impressed his manager.
Martial lasted until the hour mark before he was substituted with neither he nor Mourinho appearing to acknowledge each other as he went down the tunnel.
Man of the match - Solly March
A first for Paul Pogba - the stats
- Since taking charge of United at the start of the 2016-17 season, the only opponent Mourinho has lost against more often than Brighton (two) is his former club Chelsea (four).
- Brighton have lost just one of their past eight home games in the Premier League (W5 D2 L1), with the two most recent both being wins against United (1-0 in 2017-18 and 3-1 in 2018-19).
- United lost their first away game of a Premier League campaign for the first time since 2012-13, when they were beaten 1-0 by Everton.
- Twenty three of Murray's 24 goals in the Premier League have been scored from inside the box, including 100% of those scored for Brighton in the competition (13/13).
- Gross has scored eight goals in the Premier League, with all eight at home. Only two players have netted more goals exclusively at home in the competition's history - Georginio Wijnaldum (18) and Clive Wilson (nine).
- In competitive games since the start of the 2018 World Cup, David de Gea has conceded 10 goals from the past 14 shots on target he has faced.
- Lukaku has scored against 18 of the current 20 Premier League teams, with Cardiff and Wolves the only teams he is yet to find the net against.
- Pogba ended on the losing side in a Premier League game he scored in for the first time - prior to today, United had won each of the previous 11 games he had netted in, which was a joint-record in the competition (with Ryan Babel for Liverpool).
- This was only the 10th time in Premier League history that United conceded three first-half goals in a game, and the first time since October 2015 against Arsenal.
Brighton travel to Liverpool on Saturday, 25 August (17:30 BST) while United host Tottenham on Monday, 27 August (20:00 BST)
Home TeamBrightonAway TeamMan Utd
- Shots on Target