The two bitter foes squared off in their semi-final second leg at Atletico's Estadio Vicente Calderon on Tuesday, and Real were able to add to their 3-0 lead from the first encounter with two early Cristiano Ronaldo penalties - making it a 5-0 success overall, and sending them into the final against either Barcelona or Real Sociedad.
Ronaldo scored from the spot in the seventh and 16th minutes, with both penalties slotted into the left-hand side of the goal.
Ancelotti said having a three-goal buffer heading into the second 90 minutes was naturally beneficial.
"The advantage gained from the first leg helped to determine this evening's match," he said.
"This is an important trophy for us.
"Now we are in the final and that was an important objective.
"The Copa del Rey is important here and all teams have done their best to try to reach the final."
Ancelotti also spoke highly of Ballon d'Or holder Ronaldo, who converted with consummate ease from the spot.
"Cristiano was very good, and he scored two important goals," he said.
Ronaldo, though, was at the centre of controversy, as he was struck by a lighter thrown from the crowd at the half-time break, forcing the Portugal international to the ground after it struck him flush in the head.
The 29-year-old shook off the knock, though, and continued to play the second half before being substituted on 75 minutes.
