Twice 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, leaving Williams with a career record against her opponent of 8-0.

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, which relegated the Americans to World Group II.

"Today has been a big eye-opener," Williams said afterward. "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 taking all the majors this year.

The French has been her least successful of the four, though she won the title in Paris in 2002 and 2013.

Her doubles defeat on Sunday blotted an otherwise flawless Fed Cup record and left the U.S. facing a battle to return to its place among the elite nations next year.

"We have to work harder," U.S. captain Mary Joe Fernandez said. "We came close today and need to just keep plugging away.

"The good news is that we have a lot of players in the top 100, and hopefully, we can get two wins next year and get back into the World Group."

(Reporting by Martyn Herman, editing by Pritha Sarkar)