The 35-year-old moved up one place during a week dedicated to Fed Cup action ahead of Germany's Angelique Kerber with Czech Karolina Pliskova holding third spot.

Williams, one of the greatest players of all time, revealed Wednesday that she is expecting her first baby and will not play again this year.

Kelly Bush Novak, Williams' publicist, said this week that she "looks forward to returning in 2018" and that was echoed by her coach Patrick Mouratoglou.

Overtaking Australia's Margaret Court for the all-time record of 24 Grand Slam singles crowns -- Williams is on 23 -- is also "a determining factor" in her motivation to plough on, the Frenchman said.

Williams is due to give birth in September.