user: Question: Who was the opponent on Clay surface when partnered with Horacio Zeballos?
Columns: ['Outcome', 'Date', 'Tournament', 'Surface', 'Partnering', 'Opponents', 'Score']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Winner', '1 May 2011', 'BMW Open , Munich, Germany', 'Clay', 'Horacio Zeballos', 'Andreas Beck Christopher Kas', '7–6 (7–3) , 6–4']
SQL:

assistant: SELECT "Opponents" FROM "Table" WHERE "Surface" = 'Clay' AND "Partnering" = 'Horacio Zeballos';