CREATE TABLE table_77183 ("Outcome" TEXT, "Date" TEXT, "Championship" TEXT, "Surface" TEXT, "Opponent" TEXT, "Score" TEXT)

who is the opponent when the surface is clay, the outcome is winner and the championship is estoril on 15 april 1996?
SELECT "Opponent" FROM table_77183 WHERE "Surface" = 'clay' AND "Outcome" = 'winner' AND "Championship" = 'estoril' AND "Date" = '15 april 1996'
SELECT "Opponent" FROM "table_77183" WHERE "Championship" = 'estoril' AND "Date" = '15 april 1996' AND "Outcome" = 'winner' AND "Surface" = 'clay'