Jumping from the bench, Garnacho scored twice as Chelsea won 3-1 on the night.
The Argentina winger said afterwards: ""They (Cardiff) were a very strong team. We weren't focused on which league they play in. They did a really good job but we're happy to be in the semi-final.
"They brought me on and I tried to help the team. Me, Pedro (Neto) and the rest of the players did that. We are really happy.
"As a group we are doing really well. Sometimes you have to start games and sometimes you have to help from the bench. We don't focus on this, we just try to help the team.
"We'll try to compete for everything, and the semi-final is the next one so we'll try to win."
Pedro Neto also scored on the night and added: "It's really important. The manager said in the last 18 months this is the third semi-final we had, so we are really happy with that. We got the win, so we are already looking for the next game.
"Sometimes we were under control but sometimes it's the small details that make the difference. Maybe we lost a bit of concentration because we had a lot of corners in the time in the game, and the first time they went up after the corners, they score. So it was a bit of a lack of concentration, but in the end we could go through, so we are really happy.
"It was the concentration. It was the will of winning this game. We wanted to win a lot this game. Then we had guys on the pitch who could make the difference and Garna today helped us a lot as well with two goals.
"We are Chelsea, we want to win trophies. Our mindset is to win all the games."