What is the 1991 for 2R 1999?
CREATE TABLE "grand_slam_singles_performance_timeline" (
  "tournament" text,
  "1988" text,
  "1990" text,
  "1991" text,
  "1992" text,
  "1993" text,
  "1995" text,
  "1996" text,
  "1997" text,
  "1998" text,
  "1999" text,
  "2000" text,
  "2001" text,
  "career_sr" text,
  "career_win_loss" text
);
SELECT "1991" FROM "grand_slam_singles_performance_timeline" WHERE "1999"='2r';