What is the opponent with the outcome of runner-up with a date of 19 september 1994?
CREATE TABLE "singles_18_12_6" (
  "outcome" text,
  "date" text,
  "tournament" text,
  "surface" text,
  "opponent" text,
  "score" text
);
SELECT "opponent" FROM "singles_18_12_6" WHERE "outcome"='runner-up' AND "date"='19 september 1994';