The American, 36, has risen 153 places to 28th despite losing to Angelique Kerber in only her fourth tournament since giving birth 10 months ago.
However, Britain’s Johanna Konta has dropped to 50th in the WTA rankings.
The 27-year-old reached the semi-finals at Wimbledon last year and was ranked as high as fourth in the world immediately afterwards.
But she came into this year’s tournament ranked 24th after a poor run of form and her second-round loss to former world number four Dominika Cibulkova has seen her tumble further down the standings.
The defeat by Germany’s Kerber meant Williams, who returned to the WTA Tour in Indian Wells in March, missed out on a record-equalling 24th Grand Slam singles title.