user: Question: What is the surface of the tournament with David Nalbandian as the opponent in the final?
Columns: ['Date', 'Tournament', 'Surface', 'Opponent in final', 'Score in final']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['April 26, 1999', 'Munich , Germany', 'Clay', 'Franco Squillari', '6–4, 6–3']
SQL:

assistant: SELECT "Surface" FROM "Table" WHERE "Opponent in final" = 'David Nalbandian';