The 21-year-old scored his side's second in their 3-0 thrashing of Dortmund on Wednesday and admitted his starting berth was a pleasant surprise
Gareth Bale grabbed an early opener before Isco doubled the lead before half-time, with Cristiano Ronaldo rounding off the scoring to equal Lionel Messi's record for goals scored in a single Champions League season.
Isco was chosen to start alongside Luka Modric and Xabi Alonso after Angel Di Maria was ruled out not long before kick-off and - as well as praising his team-mates for the win - the attacking midfielder admits being chosen to start was a surprise.
"It’s an excellent result. I’m very happy that I scored and also for the clean sheet," the 21-year-old told reporters.
"Pepe and the rest of the defence performed to a very high level. So did the rest of the team. It’s a deserved victory.
"I was surprised to be called when Angel felt sick but we must always be ready to play. I enjoyed having the chance to show the coach I can play in any formation and in any position.
"I loved being applauded off by the fans at the end."
Dortmund knocked Madrid out in the semi-final stage last season but face a seemingly impossible task to repeat that feat ahead of the second leg next Tuesday.