Tell me the 1968 for 1r
CREATE TABLE "grand_slam_singles_tournament_timeline" (
  "tournament" text,
  "1966" text,
  "1967" text,
  "1968" text,
  "1969" text,
  "1970" text,
  "1971" text,
  "1972" text,
  "1973" text,
  "1974" text,
  "1975" text,
  "1976" text,
  "career_sr" text
);
SELECT "1968" FROM "grand_slam_singles_tournament_timeline" WHERE "1967"='1r';