CREATE TABLE table_name_85 (opponents VARCHAR, surface VARCHAR, date VARCHAR)

Who were the opponents that played on a hard surface on July 13, 2008?
SELECT opponents FROM table_name_85 WHERE surface = "hard" AND date = "july 13, 2008"
SELECT "opponents" FROM "table_name_85" WHERE "date" = "july 13, 2008" AND "hard" = "surface"