Real Madrid defeated Dortmund 3-2 on Saturday, with three goals coming in injury-time - including a superb overhead kick from Kylian Mbappe.
Gonzalo Garcia was again on the mark for Real, which had Dean Huijsen sent off in added-time.
Xabi said afterwards, "That's football. Until 2-1, we controlled the game quite well and had it on our side. From then on, too many things happened in too short a space of time.
"We're in the semi-finals and we're happy. Hopefully, it will serve as a lesson not to let ourselves get carried away and to be focused for the whole game. I'm happy with the game: very serious, very much a quarter-final match.
“We were dominating the game well and with the changes we took control. We even had chances to score the third. The fans were really behind us, but after the 2-1 there was some confusion and we need to look at that and improve. We have to take the positives so that what happened at the end of today doesn't happen again in the future if we find ourselves in the same position."
Gonzalo did it again
On Gonzalo, Xabi said: “I've talked a lot about him and today he did it again. He has the characteristics of a goalscorer and has had a great season with Castilla. He's in the right place at the right time. We're happy with the work he's doing and he's backing it up with goals."
Wing-back Fran Garcia also scored and Xabi added: “He's having a great tournament, with top-level concentration and intensity. He reads the game well with and without the ball and today he benefited a lot from the fact that Vini Jr. was attracting a lot of attention.
"I'm very happy for him because he's one of our own, with a great attitude and he's an example of how to prepare. I told him that if he got the chance, he could score, and today he was where he needed to be."