user: Question: What is the name of a partner that played on a Hard surface on 18 April 2011?
Columns: ['Date', 'Tournament', 'Surface', 'Partner', 'Opponent in the final', 'Score']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['2 October 2006', 'Traralgon , F10', 'Hard', 'Matthew Ebden', 'James Cerretani Phil Stolt', '6–3, 6–3']
SQL:

assistant: SELECT "Partner" FROM "Table" WHERE "Surface" = 'Hard' AND "Date" = '18 April 2011';