user: Question: What is Surface, when Opponents is "Daniel Nestor Sandon Stolle"?
Columns: ['Outcome', 'Date', 'Tournament', 'Surface', 'Partner', 'Opponents', 'Score']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Winner', '7 January 1991', 'Adelaide , Australia', 'Hard', 'Stefan Kruger', 'Paul Haarhuis Mark Koevermans', '6–4, 4–6, 6–4']
SQL:

assistant: SELECT "Surface" FROM "Table" WHERE "Opponents" = 'Daniel Nestor Sandon Stolle';