The 18-year-old Spanish-born of Ghanaian descent scored the last goal in the 81st minute to seal victory for the home side.
Oier Zarraga opened the scoring on 17 minute before Aitor Paredes increased the tally in the 74th minute.
The Ghanaian put the icing on the cake with a sublime finish with nine minutes left on the clock.
Williams has bagged two goals and provided four assists in 8 appearances so far this season.
He is the younger brother Ghana target Inaki Williams who plays for Athletico Bilbao senior side.