CREATE TABLE table_8544 ("Tournament" TEXT, "Surface" TEXT, "Week" TEXT, "Winner and score" TEXT, "Finalist" TEXT, "Semifinalists" TEXT)

Which Surface has a Semifinalist of stefan edberg thomas muster?
SELECT "Surface" FROM table_8544 WHERE "Semifinalists" = 'stefan edberg thomas muster'
SELECT "Surface" FROM "table_8544" WHERE "Semifinalists" = 'stefan edberg thomas muster'