user: Question: What is Opponent, when Round is World Group?
Columns: ['Outcome', 'Edition', 'Round', 'Opponent Team', 'Surface', 'Opponent', 'Score']
Types: ['text', 'real', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Loser', 2010.0, 'World Group Playoffs', 'Slovakia', 'Clay (i)', 'Daniela Hantuchová', '6–2, 6–2']
SQL:

assistant: SELECT "Opponent" FROM "Table" WHERE "Round" = 'World Group';