What was the surface when the opponent was judith wiesner, and the outcome was winner?
CREATE TABLE "singles_18_12_6" (
  "outcome" text,
  "date" text,
  "tournament" text,
  "surface" text,
  "opponent" text,
  "score" text
);
SELECT "surface" FROM "singles_18_12_6" WHERE "outcome"='winner' AND "opponent"='judith wiesner';