CREATE TABLE table_45698 ("Date" TEXT, "Tournament" TEXT, "Surface" TEXT, "Opponent" TEXT, "Score" TEXT)

Who was the opponent on April 21, 2008?
SELECT "Opponent" FROM table_45698 WHERE "Date" = 'april 21, 2008'
SELECT "Opponent" FROM "table_45698" WHERE "Date" = 'april 21, 2008'