user: Question: Which opponent used a Grass surface?
Columns: ['Outcome', 'Tournament', 'Surface', 'Opponent', 'Score']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Winner', 'Louisville , US', 'Hard (i)', 'Donald Young', '0–6, 7–5, 6–3']
SQL:

assistant: SELECT "Opponent" FROM "Table" WHERE "Surface" = 'Grass';