user: Question: Who was the opponent on April 21, 2008?
Columns: ['Date', 'Tournament', 'Surface', 'Opponent', 'Score']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['March 27, 2006', 'Buenos Aires', 'Clay', 'Cristian Villagrán', '4–6, 6–3, 6–4']
SQL:

assistant: SELECT "Opponent" FROM "Table" WHERE "Date" = 'April 21, 2008';