Franco Brienza fired home a stunning late winner as Atalanta beat Milan 2-1 on Sunday to dent the latter's bid for European qualification.
Milan looked set to boost their chances of securing a place in the UEFA Europa League, and salvaging something from a largely disappointing campaign, when they took the lead early in the second half.
A Gianpaolo Bellini own goal put Milan in front in the 51st minute, but the hosts drew level before claiming victory in sensational fashion.
Top scorer German Denis equalised from the penalty spot for Atalanta with 22 minutes remaining before substitute Brienza had the final say.
With virtually the last kick of the game, the 34-year-old curled a wonderful left-footed shot into the top-left corner from 30 yards, heaping more pressure on Seedorf.
Sadly, matters on the field were marred by a banana being thrown onto the pitch in the direction of Milan defender Kevin Constant, an action that was met with sarcastic applause from team-mates Philippe Mexes and Nigel de Jong.
Milan made three changes to the side that beat city rivals Inter last weekend, Marco Amelia, Sulley Muntari and Keisuke Honda coming in for Christian Abbiati, who was suspended, Andrea Poli and Adel Taraabt.
Chances proved few and far between in a first half dictated for long periods by Milan, although the hosts may have taken the lead had Carlos Carmona been able to provide a better finish midway through the first half. The Chilean's weak shot was comfortably saved by Amelia.
Atalanta again went close in the 29th minute as Denis fired wide from the edge of the area, before Maxi Moralez missed the target with a header from Luca Cigarini's free-kick.
However, the best opportunity of the first half came five minutes before the interval as Atalanta goalkeeper Andrea Consigli tipped Mario Balotelli's curling free-kick against the crossbar.
Consigli was less convincing moments later when Balotelli fired in another swerving free-kick that he could only parry, with Honda heading wide on the follow-up.
Stephan El Shaarawy came on to replace Honda at the start of the second half, the forward returning from injury to make his first appearance of 2014.
Kaka then flashed a header over the bar before Seedorf's men broke the deadlock, albeit in somewhat fortuitous fashion.
Muntari broke down the left and aimed a low cross towards Balotelli, who appeared to be in an offside position. In attempting to prevent the ball from reaching Balotelli, Bellini turned the ball into his own net.
However, the visitors failed to take control of the contest after going ahead and Atalanta levelled when Denis sent Amelia the wrong way with a confident penalty after Carmona had been tripped by Constant.
Balotelli was thwarted by the woodwork again, hammering an effort against Consigli's right-hand post as Milan pressed for a winner.
Yet there proved to be one final twist in the tale as Brienza earned Atalanta three points with a brilliant first-time shot.
|Coach: S. Colantuono|
for G. Bonaventura 69'
for D. Baselli 75'
for M. Moralez 87'
|Mkhitaryan keeping fingers crossed over injury|
|Casillas left frustrated by Alex Telles red card|
|Anything can happen at King Power Stadium – Away goal lifts Schmeichel|
|Ranieri hails Leicester´s ´big heart´ in Sevilla loss|
|Krasnodar celebrate eighth anniversary by eliminating Fenerbahce|
|Home bird Rooney not suited to China - Phil Neville|
|Ramos rejects claims of bad Real Madrid attitude|
|Porto 0 Juventus 2: Substitutes Pjaca and Alves give Bianconeri command|
|Sevilla 2 Leicester City 1: Vardy´s strike tees up second leg after Schmeichel heroics|
|Madrid lead is not worth anything – Zidane|
|Dropped Bonucci must adapt to the rules – Marotta|
|I cannot do what Man City did at Chelsea - Mourinho withering over FA Cup schedule|
|Marcelo: Real Madrid must learn from horror start at Valencia|
|Manchester United head to Wembley brimming with confidence – Smalling|
|Valencia 2 Real Madrid 1: Zaza stunner hands title hope to Barcelona|
|Manchester United duo Mkhitaryan and Carrick set to miss EFL Cup final|
|Alba: Luis Enrique the best coach for Barca|
|Sorry for that poor guy - Sane apologises after costing punter huge windfall|
|Saint-Etienne 0 Manchester United 1 (0-4 agg): Mkhitaryan on target before injury scare|
|Kessie dreaming of Manchester United move|
|Kane refuses to blame ´incredible´ Wembley for Champions League exit|
|Mkhitaryan limps out of Manchester United Europa League clash|
|De Sciglio back in AC Milan training|
|Perez ´really comfortable´ after settling at Arsenal|
|Lallana signs three-year extension at Liverpool|
|Lyon have already discussed replacing Lacazette - Aulas|
|Low pleased to avoid fatigue after Euro 2016 campaign|
|Some teams are beaten before kick-off - Hazard hails Chelsea home form|
|AFC Champions League Review: Misery for history-making Yuen-ting, Ramires boosts Jiangsu|
|Spalletti not interested in Europa League favourites tag|
|Ventura: Balotelli´s latest signs not positive|
|Vela not expecting Real Sociedad Champions League return|
|Schone renews for two years at Ajax|
|Pochettino calls for patience in Tottenham trophy hunt|
|Demirbay earns two-year Hoffenheim extension|
|Origi eyeing first team place at Liverpool|
|Tottenham legend King encourages Alli to keep his edge|
|Pochettino: Spurs must wait for Rose return|
|Romero eyes Europa League title ahead of birthday trip to Saint-Etienne|
|Mowbray tasked with saving struggling Blackburn|
|Four more years? Celtic´s Dembele not stressing amid Real Madrid links|
|RB Leipzig or Salzburg Champions League ban ´hypothetical´ - UEFA|
|Manchester United have world-class players in every position - Forster|
|James: Zidane´s rotation does not affect Real Madrid|
|Redan to snub Manchester United and sign Ajax deal|
|Giovinco turned down China move ´for now´ - agent|
|Guardiola wanted to destroy Monaco´s 4-4-2, says Fabinho|
|Sagna hails Stones after ´vital´ Manchester City goal|
|Deulofeu loves AC Milan but future ´depends on Barcelona´|
|Monaco lacked intelligence, says frustrated Falcao|
|That is why Manchester City wanted me - Guardiola relishes Champions League thriller|
|Tianjin deny Rooney bid but admit Aubameyang talks|
|Deportivo Saprissa 0 Pachuca 0: Champions League quarter-final evenly poised|
|Gameiro angered by substitution in Atletico win|
|´I give nothing for free´ – Mourinho tells Shaw to learn from Mkhitaryan|
|Champions League quarter-finals still possible for Monaco, says Silva|
|Guardiola asks me for more, more, more - Aguero happy to fight on at Manchester City|
|It was like ice hockey - Monaco vice-president stunned by Champions League clash|
|Proud Griezmann eyes more Atletico history|
|Toure marvels at Caballero heroics in Manchester City-Monaco thriller|
|Buffon hopes to play well against Casillas|
|Schmidt: It is easy to see how Atletico Madrid have reached two finals in three years|
|Falcao penalty miss was key – Jardim|
|Guardiola: If one team can score a thousand, million goals it is Monaco|
|Griezmann hails ´brilliant´ Gameiro as Atletico see off Leverkusen|
|Simeone all smiles after Atletico dismantle Leverkusen|
|Spot-kick revision pays off for Caballero|
|Championship Review: Huddersfield boost top-two hopes, Wednesday beaten|
|Toprak laments bad result as Leverkusen lose at home to Atletico|
|Aguero grapples for glory as strikers shine in Manchester City v Monaco|
|Bayer Leverkusen 2 Atletico Madrid 4: Simeone´s side in control|
|Manchester City 5 Monaco 3: Guardiola´s side prevail in all-time classic|
|AFC Champions League Review: Al Rayyan and Al Ahli off to winning starts|
|Ranieri: Sevilla game can be a turning point for Leicester|
|Allegri confirms Bonucci absence after row|
|Nuno confident Porto can beat Juventus|
|Matthaus encourages Gotze to stay at Dortmund|
|Martial: I love Manchester United and I want to stay|
|I would never force out Manchester United legend Rooney - Mourinho|
|Anderlecht insist Tielemans not leaving on the cheap|
|Leicester can´t afford to live in the past, warns Sampaoli|
|Porto´s home comforts, shot-shy Leicester & Sevilla meet - Champions League in Opta numbers|
|Puyol backs Barcelona for Champions League miracle|
|Sutton´s pie-munching keeper Shaw resigns|
|Coyle leaves struggling Blackburn|
|Sampaoli noncommittal on Sevilla future despite contract offer|
|Higuain backs Dybala to become ´among the best in the world´|
|Ancelotti escapes punishment for middle finger|
|Pie-eating Sutton goalkeeper´s antics under investigation|
|AFC Champions League Review: 2014 winners Wanderers thrashed, Hulk inspires 10 men|
|Zidane reveals Ronaldo´s favourite position|
|Lippi backs Allegri for Juve stay|
|Zidane: Isco knows I love him|
|Rooney returns to United training|
|Higuain confident Juventus can win Champions League|
|Giggs the dream pro-am partner for Manchester United fanatic McIlroy|
|Kane hopes Spurs can get used to Wembley|
|Hazard: I gave everything to come back|
|Like Cantona, Ibrahimovic embraces United pressure - Giggs|
|Wenger: Perez deserves more chances|
|Wenger denies using Sanchez was risky|
|Tite: I wish Messi was born in Brazil|
|Jardim praises ´great striker´ Falcao|
|Reported Barcelona target Sampaoli offered Sevilla extension|
|´Angry´ Messi didn´t celebrate because of criticism – Bauza|
|´He is going to have to do it himself´ – Guardiola backs Aguero|
|Toure backs Sterling and Sane for the top|
|Pique: Barcelona were in deep s*** when Luis Enrique arrived|
|Thoughts of Bayern mauling stopped Wenger from enjoying Arsenal FA Cup win|