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'
|Braafheid signs Lazio extension|
|Nani finalises Fenerbahce move|
|Liverpool confirm O´Driscoll appointment|
|Liverpool have to start better - Lucas|
|USA simply too good - Sasaki|
|Portland Timbers 1 San Jose Earthquakes 0: Jewsbury wins it in stoppage time|
|No move yet for Van Persie|
|Lloyd wins Golden Ball, Silver Boot awards|
|Mascherano: I never grabbed Banega|
|Dunga requested Brazil not appeal Neymar´s suspension|
|United States 5 Japan 2: Lloyd stars with hat-trick|
|Fiorentina deny Inter criticism|
|Nani relishing Fenerbahce opportunity|
|Newcastle hold talks over Northampton youngster|
|Cuadrado´s agent plays down Chelsea exit rumours|
|Liverpool´s Alberto set for Deportivo loan|
|Leverkusen suffer surprise defeat, Stuttgart hit six|
|Inter ´dismayed´ at Fiorentina vice-president´s Salah Twitter outburst|
|Blaszczykowski ruled out of Dortmund´s Asia tour|
|Gerrard gets the beers in for LA Galaxy fans|
|De Gea coy over Real links|
|Former Brazil star Carlos handed Delhi reins|
|Ciftci will not push through Celtic switch – agent|
|In-demand Mitrovic considering Roma offer|
|Lehmann calls on Cech to ´get over´ Chelsea|
|Marlos double sinks Monaco|
|Mauritius 0 South Africa 2 (agg 0-5): Bafana set up Angola qualifier|
|Keane relishing Gerrard arrival|
|Palace confirm Cabaye interest|
|Hamburg bring in Spahic after head-butt ban|
|Blatter hits out at France and Germany over World Cup votes|
|Sanchez: World Cup exit spurred Chile to Copa glory|
|Jojic makes Cologne switch, Santana move falls through|
|Carrillo keen to get going at Monaco|
|Micronesia thrashed 38-0 as Fiji triumph in Olympic qualifier|
|Schalke stunned by ´machine´ Caicara|
|MLS Review: Keane shines at Gerrard´s Galaxy unveiling|
|Counter-attacking the key for Japan|
|Jimenez happy at Atletico|
|Gerrard predicts ´successful future´ with Galaxy|
|Iniesta can´t see Real selling Ramos|
|Keshi sacked as Nigeria coach|
|Three finals, three defeats for Messi, Mascherano|
|Nani close to Fenerbahce move|
|Sampaoli revels in ´unbelievable joy´|
|Bravo: Copa victory a dream|
|Martino: Argentina should have won|
|Chile 0 Argentina 0 (4-1 on penalties): Hosts end trophy drought|
|Germany 0 England 1 (after extra time): England make World Cup history|
|Kenya 0 Ethiopia 0 (agg 0-2): Hosts sent out of CHAN qualifiers after second penalty miss|
|Dortmund put 17 past Olympians, Bremen hand out thrashing|
|Dinamo Zagreb confirm arrests of coach and club president|
|I didn´t know who Wimbledon guest Klopp was - Brown|
|LeBron James backs Liverpool captain Henderson for FIFA 16 cover.|
|Ramos: I am not negotiating with anyone|
|Valencia complete Mina swoop|
|United States v Japan: US seek final redemption|
|Hanousek consigns Celtic to defeat|
|Roma confirm Dzeko interest|
|Arsenal forward Podolski completes Galatasaray switch|
|Warm Dortmund welcome delights Tuchel|
|Silva vows to fight for Madrid place|
|Mancini hints at Shaqiri and Santon departures|
|Bojan raring to go after return from injury|
|Babel backs Firmino to shine at Liverpool|
|Falcao must wait to impress, warns Chelsea favourite|
|Bartomeu: Deal in place for Turan|
|Parejo re-signs with Valencia|
|MLS Review: Sounders return to winning ways|
|Klinsmann unhappy with USA performance|
|High hopes for Paraguay - Diaz|
|Gareca: More to come from Peru|
|Gerrard had European offers|
|Argentina out to spoil Chile´s party - Martino|
|Sampaoli: Chile won´t change style|
|Di Maria will come good at United - Rojo|
|Peru 2 Paraguay 0: Gareca´s men finish third at Copa|
|United States 4 Guatemala 0: Klinsmann´s men win comfortably|
|´I’d prefer no-one to know my name´ - Bassett|