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'
|Russian Premier League Review: Torpedo keep survival hopes alive|
|Turkish Super Lig Review: Galatasaray crowned champions as Fener have four sent off|
|Ghana 1 Madagascar 2: Simouri nets late winner|
|Galatasaray reclaim title as Fener implode|
|Ancelotti bears no grudge after Real sacking|
|Porto complete Bueno capture|
|Lazio 1 Roma 2: Yanga-Mbiwa settles Rome derby|
|Real sack Ancelotti|
|Route to the UEFA Europa League final: Sevilla|
|Derby sack McClaren|
|Eredivisie play-offs Review: Eagles relegated, NAC Breda earn reprieve|
|Inzaghi eager for Van Ginkel stay|
|Route to the UEFA Europa League final: Dnipro|
|Dani Alves plays down Champions League favourites tag|
|Middlesbrough 0 Norwich City 2: Jerome and Redmond send Neil´s men up|
|Renard takes over at Lille|
|Cancelo completes permanent Valencia move|
|Dnipro v Sevilla: Emery embracing chance at history|
|Kuchuk leaves Kuban Krasnodar|
|Desailly hails Terry and urges calm over Zouma|
|Guardiola has built on Heynckes´ Bayern legacy - Muller|
|Falcao thanks United fans|
|Zorniger takes Stuttgart role|
|Dani Alves to clarify Barca future after Champions League final|
|Pazzini in the dark over Milan future|
|Barzagli a doubt for Champions League final|
|Clough sacked by Sheffield United|
|Pardew eyeing ´quality´ signings|
|Carver encouraged by Ashley´s Newcastle promises|
|Gutierrez revels in Newcastle ´fairytale´|
|Relegation will haunt Hull - Chester|
|El Shaarawy eyes Italy call-up|
|Crouch delighted by headed goals record|
|Ancelotti all but confirms Real exit|
|Platini: FIFA will lack credibility if Blatter remains president|
|Bilbao duo pen new deals|
|Rooney: United can win title next season|
|Argentine football rocked by second death|
|Leicester must aim higher next season - Pearson|
|Monk negotiating new Swansea contract|
|Ox can play in cup final, says Wenger|
|Century of MLS goals ´surreal´ for Wondolowski|
|Copa Libertadores Preview: River not giving up hope|
|Cahill returns for Australia´s World Cup qualifier in Kyrgyzstan|
|MLS Review: Wondolowski joins 100-goal club|
|Toni on track for Hellas Verona´s maiden golden boot|
|Amid Ancelotti speculation, Galliani confirms Madrid trip|
|Berlusconi targets Ancelotti, Inzaghi remains committed to Milan|
|Serie A Review: El Shaarawy at the double in Milan victory, Fiorentina edge five-goal thriller|
|Taylor close to signing new Swansea contract|
|Milan 3 Torino 0: El Shaarawy grabs brace as Inzaghi´s men ease to victory|
|Bruce expresses sympathy with Hull owner and fans following relegation|
|Preston North End 4 Swindon Town 0: Beckford hat-trick seals promotion|
|Darmstadt promoted back to the Bundesliga after 33-year exile|
|Sherwood to assess Given injury ahead of FA Cup final|
|Europa League qualification would cap ´special´ season - Koeman|
|South Africa 0 Botswana 0 (6-7 on penalties): Hosts exit COSAFA Cup at first hurdle|
|Pearson hopeful of Cambiasso stay|
|Ings emotional after victorious ´last game´|
|Pochettino happy to finish fifth|
|West Brom release Mulumbu|
|Monk: Palace defeat not a reflection on season|
|LVG awaits news on ´minor´ Di Maria injury, refuses to comment on Falcao exit reports|
|Pulis accepts blame for West Brom´s Arsenal collapse|
|Toure and Milner were not saying goodbye - Pellegrini|
|Ramsey calls for patience in QPR rebuild|
|Allardyce: Time is right for West Ham exit|
|Mourinho already preparing for title defence|
|Eredivisie Review: Heerenveen to play Vitesse for Europa League place|
|Hughes hails Stoke showing|
|Carver still hoping to land Newcastle job|
|Sunderland squad needs better balance, says Advocaat|
|Pardew ´never dreamt´ of top-half finish at Palace|
|Survival Sunday: How the final-day Premier League relegation battle played out|
|Arsenal wanted Walcott stay regardless of hat-trick - Wenger|
|Bruce: Hull have not been good enough|
|De Gea has to say something - Van Gaal|
|Rodgers apologises after ´embarrassing´ defeat|
|Chelsea 3 Sunderland 1: Remy double secures win as Drogba bids farewell|
|Stoke City 6 Liverpool 1: Gerrard nets consolation in final-game thrashing|
|West Ham confirm Allardyce exit|
|Newcastle United 2 West Ham 0: Gutierrez on target as Carver´s men stay up|
|Crystal Palace 1 Swansea City 0: Chamakh earns season-closing win|
|Manchester City 2 Southampton 0: Lampard bids Premier League farewell with goal|
|Hull City 0 Manchester United 0: Bruce´s men relegated|
|Aston Villa 0 Burnley 1: Ings heads winner|
|Everton 0 Tottenham 1: Kane header seals fifth place|
|Leicester City 5 QPR 1: Vardy celebrates England call-up in style|
|Arsenal 4 West Brom 1: Wilshere screamer and trio for Theo secures third|
|Drogba carried from the pitch by Chelsea team-mates|
|Scottish Premiership Review: United take Dundee derby spoils, Celtic smash Inverness|
|Ashley not leaving Newcastle until club wins something|
|Klopp urges fans not to draw comparisons with Tuchel|
|Dortmund complete Castro capture|
|Stevens leaves Stuttgart after securing safety|
|Alves ruled out of Copa with serious knee injury|
|Drogba to leave Chelsea|
|Ashley is from another planet - Keegan|
|Sydney withdraw Janko offer amid talk of Toni|
|Cameron pens Stoke extension|
|Wimmer: I´m going to Tottenham|
|Returning Grabban ready for play-off final - Neil|
|Ferdinand ´overwhelmed´ by support following wife´s death|
|Odegaard enjoys ´nervous´ Real Madrid debut|
|Bamford fit but may be overlooked for play-off final|
|Barcelona´s best better than Real´s - Luis Enrique|
|Ancelotti laments key injuries in title failure|
|AFC Champions League Preview: Evergrande, Al Hilal desperate for wins|
|MLS Review: DC United deny nine-man Revolution, Rapids break drought|
|Bielsa hints at Marseille departure, congratulates Ligue 1|
|Silva, Matuidi eye fourth trophy|
|Jonas deserved golden boot - Jorge Jesus|
|Primeira Liga Review: Benfica celebrate title with home win|
|Ligue 1 Review: Monaco seal final Champions League spot, Marseille take fourth|
|Ancelotti to take sabbatical if sacked by Real|
|Mancini annoyed by ´absurd´ Inter mistakes|
|Simeone hails ´incredible season´|
|Replacing Xavi will be impossible - Luis Enrique|
|Ronaldo hopes Ancelotti will stay|
|Guardiola: We´ll be back stronger|
|Paris Saint-Germain 3 Reims 2: Cavani double sees champions celebrate title with routine win|
|Garitano stands down following Eibar relegation|
|Genoa 3 Inter 2: Kucka strikes blow to visitors´ Europa quest|
|Real Madrid 7 Getafe 3: Ronaldo nets treble and Odegaard makes history|
|Southend United 1 Wycombe Wanderers 1 (aet, 7-6 on penalties): Last-gasp Pigott and Bentley the hero|
|Odegaard becomes Real´s youngest player|
|Gargano omitted from Uruguay Copa America squad|
|La Liga Review: Depor save themselves with stunning comeback at Barca|
|Barcelona 2 Deportivo La Coruna 2: Salomao saves visitors as Xavi bids Camp Nou farewell|
|Granada 0 Atletico Madrid 0: Atleti secure third as hosts survive|
|Juventus 3 Napoli 1: Champions turn on the style|
|Tearful Klopp struggled to say goodbye|
|Scottish Premiership Review: Rangers to meet Motherwell in play-off final|
|Bundesliga Review: Freiburg and Paderborn relegated|
|Borussia Dortmund 3 Werder Bremen 2: Dortmund seal European spot as Klopp says farewell|
|Bayern Munich 2 Mainz 0: Schweinsteiger nets as champions celebrate title|
|Championship play-off final Preview: Karanka and Neil eye swift ascent to Premier League|
|Volland commits to Hoffenheim|
|Monk: Michu likely to move on|
|Allardyce not interested in Newcastle revenge|
|Sherwood confident of Benteke Villa stay|
|Rodgers calls on Liverpool fans to support Sterling|
|Khedira ´frozen out´ by Real|
|Hamstring injury rules Suarez out of Deportivo clash|
|Former Southampton managers hail ´excellent season´|
|Mourinho wants to remain in England after Chelsea|
|Rodgers ´150 per cent sure´ his job is safe|
|Maldini would ´love´ to help Milan|
|Doyle set for Rapids debut versus Whitecaps|
|MLS Review: Champions end winless run on day of late drama|
|Jemez doubted Rayo future|
|Zenga could replace Mihajlovic - Osti|
|Blanc wants to avoid Chelsea-like finish|
|Menez happy in Milan|
|Sherwood: Vlaar wants to stay|
|Arsenal v West Brom: Wenger calls for focus|