If the opponent is Kristian Pless, who was it against?
CREATE TABLE "singles_performances_18_10" (
  "edition" text,
  "round" text,
  "date" text,
  "against" text,
  "surface" text,
  "opponent" text,
  "result" text,
  "outcome" text
);
SELECT "against" FROM "singles_performances_18_10" WHERE "opponent"='Kristian Pless';