user: Question: WHich Surface has an Opponent of Gustavo Kuerten?
Columns: ['Date', 'Tournament', 'Surface', 'Opponent', 'Score']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['29 September 1997', 'Palermo , Italy', 'Clay', 'Alberto Berasategui', '4–6, 2–6']
SQL:

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