Gylfi Sigurdsson's smart finish saw Swansea City claim a surprise 2-1 win at Manchester United as the Louis van Gaal era started in defeat.
Sigurdsson bagged the winner 18 minutes from time on Saturday as Van Gaal – tasked with improving the fortunes at Old Trafford after David Moyes' disastrous spell in charge last term – endured a day he will want to forget in his opening Premier League fixture.
The Dutchman's 3-4-1-2 system did not appear to suit United despite being used in pre-season and they found themselves behind when Ki Sung-yueng scored from 20 yards just before the half-hour mark.
Van Gaal, known for his tinkering, swapped to 4-4-1-1 in the second half and it appeared to be the tonic for United's response, led by new captain Wayne Rooney, who levelled with a clever overhead kick eight minutes after the break.
Rooney saw an outstanding free-kick crash against the post with 24 minutes left and Old Trafford was then stunned when Sigurdsson – Swansea's main signing of the close-season – finished well after Wayne Routledge's cut-back.
The result is sure to put early pressure on Van Gaal and is likely to see further spending from United in the transfer window, as they look to recover from last term's seventh-place finish.
The victory was Swansea's second at Old Trafford in 2014 – and second ever - as the Welsh club also won a third-round FA Cup tie by the same scoreline in January.
Van Gaal gave 20-year-old defender Tyler Blackett a Premier League debut, while Spanish midfielder Ander Herrera and Jesse Lingard also made their top-flight bows.
Lingard started in the unfamiliar position of right wing-back and was nearly caught out just six minutes in, when Routledge drifted across him untracked, but the Swansea man's first touch meant that Jonjo Shelvey's clever ball in behind was squandered.
Sigurdsson – back at Swansea for a second spell, this time permanently, after signing from Tottenham – saw his 25-yard shot in the 12th minute fall awkwardly for United goalkeeper David de Gea, but the Spaniard did well to smother it.
Lingard was then injured in a challenge with Ashley Williams, and replaced by Adnan Januzaj, before the hosts eventually got going, with Lukasz Fabianski – the only debutant for the visitors from the start – needing to be alert to keep out Phil Jones' deflected cross and Juan Mata's inswinging effort.
Swansea were able to convert one of their chances, though, as Ki delighted the travelling fans in the 28th minute.
The South Korea international was played in by Sigurdsson, who initially linked with Nathan Dyer, and he hit a sweet left-footed effort that curled into the right corner from 20 yards.
Van Gaal's men failed to mount a significant response before the break and as a result, he replaced the ineffective Javier Hernandez with Nani and changed systems.
Rooney then produced the equaliser eight minutes into the second half, showing great anticipation to thrash Jones' near-post flick-on home from four yards in sensational style.
The England star continued to threaten, with Williams required to deny him with a last-gasp tackle, before he was then thwarted by the woodwork as his wicked 25-yard set-piece left Fabianski rooted to the spot, but crashed back off the post.
And Swansea made the hosts pay for their profligacy as a quick free-kick, taken by Wilfried Bony, set them free.
A cross from substitute Jefferson Montero made its way through to Routledge at the far post, and although he slipped, his ball fortunately fell for Sigurdsson, who hammered an effort through a sea of bodies that was too hot to handle for De Gea.
United, so often saved by late goals, rarely looked like levelling, but were denied a penalty despite Januzaj's cross striking the arm of Angel Rangel in injury time, as Swansea claimed an excellent win against the odds.
|Coach: L. van Gaal|
for J. Lingard 24'
for J. Hernández 46'
for Ander Herrera 67'
|MLS Review: Wright-Phillips double inspires Red Bulls to New York derby rout|
|James seeks more game time at Madrid|
|Rooney: I want to be a manager|
|Mignolet welcomes Karius challenge in Liverpool number one battle|
|Batshuayi buoyed by winning vibes at Chelsea|
|Moyes and Sunderland players diverted after plane fault|
|´Loved´ Varane targets long Madrid stay|
|Kostic set for Hamburg medical|
|Leicester crying over Kante sale - Ranieri|
|Schurrle the perfect Mkhitaryan replacement - Tuchel|
|West Ham face anxious wait on Cresswell injury|
|Mourinho will accept criticism from United legends|
|Bertolacci desperate for fresh start after underwhelming first season at Milan|
|Hoops hand Toure one-year deal|
|Origi ready to fight for Liverpool spot|
|Emery wants PSG stay for ´fantastic´ Matuidi|
|No Barcelona offer for Vietto - Cerezo|
|Burnley apologise for racist abuse from supporter|
|Guardiola will not rush Kompany comeback|
|Of course I´ll shake Guardiola´s hand, snaps riled Mourinho|
|Watzke: No more signings for Dortmund|
|Roma expect Manolas to snub Premier League move|
|Adriano set to leave Barcelona for Besiktas|
|Manchester United in worst possible situation - Mourinho|
|Sporting would have to sell stadium to afford Van Persie - Jesus|
|Jonas´ agent confirms Barcelona approach|
|No problem with Mourinho, insists Guardiola ahead of derby in China|
|Toronto´s hat-trick hero Giovinco: I´m back|
|Drogba revels in ´special night´, dedicates hat-trick to family|
|MLS Review: Hat-tricks for Drogba and Giovinco as stars shine|
|Whelan backs Liverpool for top-four finish|
|Pellegrini still waiting for right offer|
|Ronaldo deserves to win Ballon d´Or - Vazquez|
|Phelan wants Hull job|
|Alaba saddened by Gotze exit|
|Morgan: Vardy right to re-sign|
|Maradona ´hurting´ as Higuain closes on Juventus move|
|Moyes watches Sunderland win, Ibe opens account in six-goal Bournemouth thriller|
|Messi cuts short holiday for Barcelona training camp|
|Ranieri still confident of keeping Mahrez|
|Toure to complete Celtic move on Sunday, says Rodgers|
|Vietto sends Simeone a reminder amid Barcelona links|
|Celtic 1 Leicester City 1 (5-6 on penalties): Mahrez magic the highlight as Ranieri´s side triumph i|
|Ribery, Alaba on target in Bayern win|
|Neymar hails Micale ahead of Olympic tilt|
|Ronaldo wants Real Madrid renewal|
|Ochoa joins Granada from Malaga|
|Heidel quashes Meyer to Liverpool as ´nonsense´|
|Scholes challenges Allardyce to change England mentality|
|Tenerife star Crosas investigated for killing endangered shark|
|De Gea: Manchester United can win title under Mourinho|
|Marseille replace Batshuayi with brother Leya Iseka|
|Cristiano Ronaldo: Best season of my career|
|Scholes: Pogba is not worth £100million|
|Gotze has not forgotten how to play football - Riedle|
|Delph learning more than ever under Guardiola|
|Jovetic ´not worried´ about Juventus|
|Allardyce wants Euro 2016 woes to fire his England|
|Giggs not in a rush to take manager´s job|
|Lopetegui: If not for Spain I would be Wolves boss|
|Shaw one of best left-backs in world - Herrera|
|Injured Sanchez to miss Arsenal tour|
|Melbourne Victory 1 Juventus 1 (Melbourne win 4-3 on pens): Thomas inspires Victory as Juve pay pena|
|Janssen: Ajax and PSV were never going to pay as much as Tottenham|
|Bayern to make Munich shooting tributes|
|Wenger wishes luck to old foe Allardyce|
|United defender Varela joins Frankfurt on loan|
|Sunderland appoint Moyes as Allardyce replacement|
|Javi Martinez devastated by Munich shootings|
|Rooney: I´m desperate to play for Mourinho, just like under Ferguson|
|Godin: Champions League the dream, but LaLiga the focus|
|Klopp hails Wijnaldum´s ´football smartness´|
|Klavan ready to fight for starting spot after completing ´dream´ Liverpool move|
|Real Salt Lake 1 San Jose Earthquakes 1: Mulholland salvages point|
|Madeira airport to be named after Cristiano Ronaldo|
|Villar, Ceferin and Van Praag announce UEFA presidential bid|
|Witsel has offers from England and Italy|
|Tuchel will bring out Gotze´s best - Watzke|
|Swansea confirm interest in Liverpool´s Allen|
|Arsenal´s Coquelin: LeBron v Curry is like Messi v Ronaldo|
|Rooney backs Allardyce appointment|
|Bayern and Ancelotti respond to Munich attacks|
|West Brom friendly cancelled over violence concerns|