Donnarumma howler costs Milan
AC Milan suffered a second successive defeat as Sampdoria ignited the UEFA Champions League race in Serie A once more with a 1-0 home win, gifted by a Gianluigi Donnarumma howler.
The Rossoneri's 10-match unbeaten run in all competitions was ended before the international break as bitter rival Inter Milan won at San Siro, and Gennaro Gattuso's men were beaten again here.
A dreadful Donnarumma error handed Gregoire Defrel a first-minute opener and the visitors never recovered on a difficult evening in Genoa.
Milan remains fourth but, ahead of Monday's (AEDT) fixtures, fifth-placed Roma is four points back and has played a game less, while Lazio, a further two points behind its city rival, has two matches in hand.
Eighth-placed Samp is six points behind Milan.
Sampdoria led after just 33 seconds when Donnarumma's weak pass inexplicably teed up the onrushing Defrel for a first-time finish into a gaping goal from just outside the six-yard box
The goalkeeper was more impressive as he turned Fabio Quagliarella's low drive around the post, however, before Emil Audero expertly parried Suso's curling effort away at the other end.
Defrel volleyed over the top from a glorious Nicola Murru cross early in the second half, while Suso continued to pose a threat for Milan and his deflected effort wrongfooted Audero and bounced just wide.
Quagliarella, chasing his 22nd league goal of the season, saw a powerful strike deflected against the crossbar and Defrel shot straight at Donnarumma, but Samp was made to settle for a narrow victory.
Milan kept chasing until the dying seconds but Krzysztof Piatek was first denied a penalty after a lengthy VAR (video assistant referee) delay and then directed a header straight at Audero.