For what round was the opponent mercedes paz with a result of 4–6, 6–1, 6–3?
CREATE TABLE "singles_21_5" (
  "edition" text,
  "round" text,
  "date" text,
  "surface" text,
  "opponent" text,
  "result" text
);
SELECT "round" FROM "singles_21_5" WHERE "opponent"='mercedes paz' AND "result"='4–6, 6–1, 6–3';