The Dutchman opened up the game after coming off the bench for the final 20 minutes to replace Pedro, providing the assist for Lionel Messi's opener in the 2-0 win
The Dutch international came off the bench to replace Pedro with 20 minutes to go and needed only five minutes to set up Lionel Messi's opener with a fine run and cross from the right.
Afellay told NOS: "This really is an incredible feeling. This is the kind of stuff you dream about when you're a kid. Making the difference in such a huge game between two arch rivals is simply fantastic. It was a nice individual action.
"Altogether, we put in a good performance. We were already the better team before Pepe's dismissal and enjoyed a lot of possession. We didn't let them provoke us and picked up a superb result."
Barcelona will attempt to defend their two-goal lead when the two sides meet again at Camp Nou next Tuesday. The winner of the tie will take on either Manchester United or Schalke in the final.