CREATE TABLE table_6381 ("Date" CLOB, "Tournament" CLOB, "Surface" CLOB, "Opponent" CLOB, "Score" CLOB)

On what surface was the opponent Michael Berrer?
SELECT "Surface" FROM table_6381 WHERE "Opponent" = 'michael berrer'
SELECT "Surface" FROM "table_6381" WHERE "Opponent" = 'michael berrer'