user: Question: Which Surface has a Semifinalist of Stefan Edberg Thomas Muster?
Columns: ['Tournament', 'Surface', 'Week', 'Winner and score', 'Finalist', 'Semifinalists']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Indian Wells', 'Hard', 'March 1', 'Jim Courier 6–3, 6–3, 6–1', 'Wayne Ferreira', 'Michael Chang Alexander Volkov']
SQL:

assistant: SELECT "Surface" FROM "Table" WHERE "Semifinalists" = 'Stefan Edberg Thomas Muster';