Ambrosini warns Milan would be "making a terrible mistake" if they thought the victory was enough to eliminate Barcelona.
"It was a game we had to play like this," he said of the defensive approach at San Siro.
"We were very careful and allowed Barça very little. It is only the first half of the tie, as going there thinking we've done something definitive would be a terrible mistake.
"It was a great achievement, but we have to consider the 180 minutes. We'll enjoy this evening, then think about the Derby.
"Of course if we had lost 3-0 then we'd have less chance of qualifying, so naturally we have a chance."