Chris Evert, arguably the greatest clay court player of all time, won the French Open seven times. However, in 1976, 1977 and 1978--right in the middle of her streak--she opted not to play at Roland Garros. She was preoccupied with World Team Tennis at the time. She also chose not to play much at the Australian Open because she didn't want to be away from her family at Christmas. "Don't think I don't think about that now," Evert recently told ESPN about her decision to skip those three French Open tournaments. "Nobody was counting titles like they are now."
Assuming that Evert would have won at least two of three she skipped, that would have brought her Grand Slam total to twenty, and a couple more Australian Open wins would have tied her with Steffi Graf for most Grand Slams won.
Evonne Goolagong, Evert's great contemporary, also made a point of not playing all of the Slams; she just had other things she wanted to do.
Things have changed a lot since Evert and Goolagong played.
Evert's win-loss record at the French Open, by the way, was 73-6, and she holds the best career win-loss record (.900) of any player, woman or man, in history.