user: Question: What was the surface for the opponent Roger Federer?
Columns: ['Date', 'Tournament', 'Surface', 'Opponent', 'Score']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['January 14, 1996', 'Auckland , New Zealand', 'Hard', 'Brett Steven', '6–4, 6–4']
SQL:

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