When was the 1991 world group I with the opponent of Li Fang?
CREATE TABLE "singles_21_5" (
  "edition" text,
  "round" text,
  "date" text,
  "surface" text,
  "opponent" text,
  "result" text
);
SELECT "date" FROM "singles_21_5" WHERE "edition"='1991 world group i' AND "opponent"='li fang';