(REUTERS) - Two-time French Open champion Serena Williams said her struggle to beat Sara Errani in the Fed Cup on Sunday had been a real "eye-opener" as the claycourt season gets into full swing.
World No. 1 Williams eventually prevailed 4-6 7-6(3) 6-3 against the dogged Italian to take her career record over her to 8-0 but the American was not impressed.
The U.S. were beaten 3-2 as Williams and Alison Riske were thrashed 6-0 6-3 in the doubles rubber by Errani and Flavia Pennetta, meaning they were relegated to World Group II.
"Today has been a big eye opener," Williams said afterwards. "I'm totally not as ready for the claycourt season as I thought I was.
"Now I'm in the mindset 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."
Williams, 33, won her 19th singles grand slam at the Australian Open and her dominance has raised talk of her claiming all the majors this year. The French Open has been her least successful of the four though despite claiming the title in Paris in 2002 and 2013.