user: Question: What surface was the tournament played on against Steven Diez?
Columns: ['Date', 'Tournament', 'Surface', 'Opponent in the final', 'Score']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['18 June 2007', 'Ireland F1', 'Carpet', 'Colin Ebelthite', '2–6, 3–6']
SQL:

assistant: SELECT "Surface" FROM "Table" WHERE "Opponent in the final" = 'Steven Diez';