Williams' comments come after her hard-fought 4-6, 7-6 (3), 6-3 victory over Sara Errani in the Fed Cup. Despite winning that match, the United States still succumbed 3-2 to Italy and were subsequently demoted to World Group II.
Reflecting on her struggle to see off the challenge of Errani, Williams admitted there was work to be done.
"Today has been a big eye-opener," Williams said during a press conference. "I'm totally not as ready for the clay-court season as I thought I was. Now I'm in the mind set of, 'You know what, I'm not on hard court.'
"I'm playing like I'm on hard court and I'm not. So I have to play and be ready to hit a thousand shots if necessary."
With the French Open hovering over the horizon, Williams knows that she will have to be at her best if she wants to win it for the third time in her career.
Williams will also be wary of the fact that the French Open has been her least successful Grand Slam, even though she won the tournament in 2002 and 2013.