The 21-year-old scored his side's second in a 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 halftime, 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 kickoff and - as well as praising his teammates 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 defense 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 semifinal stage last season but faces a seemingly impossible task to repeat that feat ahead of the second leg next Tuesday.