Last month, Mr Federer hit out at the way the WTA rankings were calculated, saying a player who has never won a Grand Slams (Jelena Jankovic) should not be granted the No.1 spot.
Marcelo Rios also had reach YE No.1 without winning any GS too, RF did not comment on that.

ATP & WTA ranking use exactly the same point allocation system.
Federer probably mentioned of his opinion based on the history of tennis # 1 female or male players, although there r exceptions, majority of them who had ranked for a long time won Slams. Rios was one of them, so was Jelena. Jelena worked hard to reach # 1 ranking, so she deserved it, IMO. Yes, it is more difficult to win a Slam, for u play for 2 weeks & 128 players participating.But the # 1 status is according to the points u have accumulated, & fair.

l disagree with Federer. They r many excellent players that never won a slam or won it once or just few times, i.e Gabriela Sabatini won it once & she was a great player. Capriati won one of her slams against Venus when a crucial point was given to her by a racist chair Umpire that then was suspended for her mistake. The ball was clearly out & overruled by the chair umpire as in & that would have given Venus a break to win the match. Demintieva never won it, but she is a better player than Sharapova who won the slam 3 times & 2 times out of those 3 was by an easy drawer in her half(i think it was fixed for her to win). Winning a slam could be due to an easy draw, luck & many other reasons. It does not proof that u deserve the # 1 spot or that u r a great player.