Luis Muriel's penalty gave Sampdoria a shock 1-0 win over AC Milan 1-0 on Sunday to compound the Rossoneri's hopes of a return to European football.
Milan, slipping to their third consecutive league defeat, remain in eighth place but are now 14 points behind leaders Juventus, who have two games in hand.
More importantly, Milan sit 10 points behind Roma in the third and last Champions League qualifying spot. The capital side can stretch their lead with a win over Fiorentina on Tuesday.
AC Milan coach Vincenzo Montella had raised hopes this season that the seven-time European champions could battle for a Champions League spot.
Milan last graced the competition in 2014 when they lost 5-1 on aggregate to Atletico Madrid in the last 16.
Former Everton striker Gerard Deulofeu spurned a great chance to break the deadlock after the restart when he fired on to the far post from seven yards out.
Milan striker Carlos Bacca had desperate penalty appeals waved away when he tumbled after a nudge by Matias Silvestre in the area.
But when Gabriel Paletta hauled dwn Fabio Quagliarella on the counter, the referee pointed immediately to the spot. Muriel sent goalkeeper Gianluigi Donnarumma the wrong way to give Samp a 70th minute lead.
Samp moved up two places to 10th at 21 points off the pace. Juventus saw their lead cut to three points when Napoli routed Bologna 7-1. But the Turin giants can pull six points clear with a win against Inter later Sunday.