"I think I need four to six weeks of sitting and doing nothing, at least two weeks of just sitting down," Williams told reporters.
The 23-time Grand Slam singles champion suffered the injury during her U.S. Open semifinals loss to Victoria Azarenka less than three weeks before her withdrawal from Paris.
"It wasn't my ankle, it was actually my Achilles," Williams said at the time. "It just overstretched. It was pretty intense."
She skipped the Italian Open to give herself some additional rest before the season's final major, but apparently it wasn't enough to get her back to full strength.
Williams added Wednesday she "more than likely" won't play another tournament in 2020 because she's "struggling to walk, which is a telltale sign that I should try to recover."
The 39-year-old four-time Olympic gold medalist owns the most Grand Slam titles in the Open Era and is one away from tying Margaret Court for the most in history at 24.
She's lost four consecutive major finals, however, and didn't advance beyond the semifinals in any of the three Grand Slam tournaments this year. Wimbledon, which Williams has won seven times, was canceled because of the coronavirus pandemic.
Williams may not return to action until the 2021 Australian Open, which is scheduled to begin Jan. 18 to kick off next year's major schedule.
Meanwhile, Pironkova advances to face the winner of the all-Czech matchup between Barbora Strycova and Barbora Krejcikova in the third round.