A late goal by defender Luca Caldirola earned promoted side Monza a shock 1-0 win at Inter Milan in Serie A on Saturday as Simone Inzaghi's men slumped to a fourth loss in their last five league outings.
Inter, who are on course for a first Champions League last-four appearance since 2010 following their 2-0 win at Benfica in their quarter-final first leg on Tuesday, needed three points to return to the top four.
They now remain fifth on 51 points from 30 games, two points behind fourth-placed AC Milan who were held to a 1-1 draw by Bologna earlier on Saturday.
For Inter, who recently fell to three consecutive league losses against Spezia, Juventus and Fiorentina, and drew 1-1 last weekend at Salernitana, it was their third home loss in a row for the first time in 10 years.
Inter forward Joaquin Correa could have made it 1-0 in the 24th minute when he concluded a pair of one-twos with a low shot from inside the box, only to be brilliantly denied by the alert Monza keeper Michele Di Gregorio.
After a first half short of clear chances and quality, Inter gradually pushed for a goal and Di Gregorio pulled off another superb save to deny Romelu Lukaku's header shortly before the hour mark.
The Belgium striker had one more chance a minute later but his curled shot from the edge of the box went wide.
In the 64th, Monza were close to finding a goal through almost an identical strike by forward Dany Mota.
The visitors took a surprise lead at the San Siro in the 78th minute through Caldirola, who met a corner and nodded home from close range, with the ball bouncing off the ground into the top left corner.
Substitute Lautaro Martinez was Inter's best bet to look for an equaliser but one of his closest attempts was denied by the goalkeeper and another went inches wide of the left post.
Heroic Monza, who held Inter to a 2-2 draw in the reverse fixture in January, moved up one place to 12th on 38 points.
They next host Fiorentina on April 23 when Inter travel to 14th-placed Empoli.
