What player played women's singles when marcus ellis heather olver played mixed doubles?
CREATE TABLE "21st_century" (
  "year" real,
  "men_s_singles" text,
  "women_s_singles" text,
  "men_s_doubles" text,
  "women_s_doubles" text,
  "mixed_doubles" text
);
SELECT "women_s_singles" FROM "21st_century" WHERE "mixed_doubles"='marcus ellis heather olver';