AC Milan earned their first Champions League point of the season with a 1-1 draw against Porto at San Siro on Wednesday, but the result left the seven-time European champions on the brink of elimination.
Luis Diaz struck after six minutes to put the Portuguese side in front and they dominated the first half against an under-par Rossoneri side.
An own goal from Chancel Mbemba drew Milan level after the break and Zlatan Ibrahimovic had a goal ruled out for offside, as the Italians' wait for a victory goes on.
"It's tough against teams who are used to Champions League football like Porto or Atletico. Experience is important," Milan striker Olivier Giroud told Canal Plus.
"We are a young team but I don't want to make excuses. We also needed a little more luck. We have played some good matches."
It is the first time in Milan's history that they have failed to win any of their opening four Champions League games.
Porto rose to second place with five points, four ahead of bottom side Milan.
Rossoneri coach Stefano Pioli sprang some surprises by leaving regular starters like Ibrahimovic, Franck Kessie and Simon Kjaer on the bench, possibly with an eye towards Sunday's crucial Serie A derby against rivals Internazionale.