CREATE TABLE table_9189 ("Date" CLOB, "Tournament" CLOB, "Surface" CLOB, "Partner" CLOB, "Opponents" CLOB, "Score" CLOB)

Who were the opponents that played on a hard surface on July 13, 2008?
SELECT "Opponents" FROM table_9189 WHERE "Surface" = 'hard' AND "Date" = 'july 13, 2008'
SELECT "Opponents" FROM "table_9189" WHERE "Date" = 'july 13, 2008' AND "Surface" = 'hard'