If the date is 7-21-2006, who was the opponent?
CREATE TABLE "singles_performances_18_10" (
  "edition" text,
  "round" text,
  "date" text,
  "against" text,
  "surface" text,
  "opponent" text,
  "result" text,
  "outcome" text
);
SELECT "opponent" FROM "singles_performances_18_10" WHERE "date"='7-21-2006';