user: Question: Which Opponent has a Surface of Hard on 28 August 1993?
Columns: ['Outcome', 'Date', 'Tournament', 'Surface', 'Opponent', 'Score']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Winner', '4 November 1990', 'Brentwood, Tennessee , USA', 'Hard', 'Susan Sloane', '6–3, 7–6']
SQL:

assistant: SELECT "Opponent" FROM "Table" WHERE "Surface" = 'Hard' AND "Date" = '28 August 1993';