Inter Milan bounced back after two consecutive losses in Serie A with a crucial Champions League 1-0 win against Barcelona on Tuesday night, thanks to a Hakan Calhanoglu goal.
The Turkish midfielder slotted home a clinical strike from just outside the area, with the ball going into the corner to the goalkeeper's right in added time before the break.
The home win lifted Inter to second place in Group C on six points, three behind leaders Bayern Munich and three in front of Barcelona.
Inter and Barca will face each other again next week at the Camp Nou when a third loss in four games would leave Barca in danger of being knocked out of the Champions League at the group stage for the second consecutive season.
Inter came into the match having lost four of their previous six games in all competitions and are ninth in Serie A, four points from the European qualification spots and eight behind leaders Napoli and Atalanta.
Barcelona, by contrast, arrived as the new LaLiga leaders and with striker Robert Lewandowski on a roll, having scored in six consecutive LaLiga match weeks.
But the Polish star forward made no impression at the San Siro, ending with no shots on target, having been neutralized by Inter's five-man defence.
In the 24th minute, Barcelona got a break when a potential penalty for a handball by Eric Garcia inside the box was ruled out by the VAR check which spotted a close offside in the build-up.
Inter Milan had a Joaquin Correa goal ruled out in the very next play for another offside.
However, they manage to break the deadlock at the end of the first half with Calhanoglu's precise, low shot.
Ousmane Dembele was the lone bright spot for Barcelona up front, creating the few opportunities that Barcelona had despite dominating the ball possession.
The French winger had two cross-shots that went just wide in the first half and thundered a close-range shot off the right post midway through the second half.
Dembele kept insisting until he placed a perfect cross in Pedri's path but Barca's potential equaliser was ruled out after the VAR check spotted a handball by Ansu Fati right before Pedri's strike.
Barca pressed with long balls and crosses in the final minutes. Their players clamoured for a penalty in added time, complaining about a possible handball inside the box by substitute Denzel Dumfries. But after a VAR check the referee ruled there had been no infringement by the Dutch defender.
"I'm really angry, I just can't understand how that is not a penalty; it's clear. It's an injustice," a frustrated Xavi Hernandez told Movistar Plus.
"That's why I said yesterday (at the news conference) that the referees should come out and explain their decisions, because we don't understand anything."