user: Question: On what surface was the opponent Michael Berrer?
Columns: ['Date', 'Tournament', 'Surface', 'Opponent', 'Score']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['September 3, 2001', 'Aschaffenburg , Germany', 'Clay', 'Martin Verkerk', '7–6, 6–2']
SQL:

assistant: SELECT "Surface" FROM "Table" WHERE "Opponent" = 'Michael Berrer';