Serena Williams suffered her first defeat of the year on Sunday when she was beaten 6-3, 6-4 by Justine Henin-Hardenne in the final of the Family Circle Cup.
The world number one, who completed her 'Serena Slam' in January when she won the Australian Open to hold all four Grand Slams at the same time, surrendered a 21-match winning streak with defeat at the $1.3 million tournament.
"Today was an off day and you can't be on your off day against a top player," Williams told reporters.
"She (Henin-Hardenne) played really excellent today. She had a good plan going out there and it all worked.
"She was on today and she was really fighting. She did a great job."
Having suffered her previous loss at the season-ending WTA Tour Championships last November -- to Henin's fellow Belgian Kim Clijsters -- the American had entered the final as the overwhelming favourite against a player she had beaten four times in five meetings.
But a determined Henin-Hardenne came out fighting on Sunday to upset the formbook and tame a rather subdued Williams.
The top seed's usual armoury of winners seemed to have deserted her as she erratically sprayed her shots wide and surrendered the match on her serve with a weak backhand into the net.
Williams had been hoping to win her fourth consecutive event of the year but instead had to see a jubilant Henin-Hardenne throw her arms up in the air to celebrate her second victory of the season.
"It's still very tough to beat Serena and today I just kept fighting and finally won the match," said the 20-year-old Belgian, who also triumphed in Dubai in February.
"She's number one in the world and she deserves that so much. But I agree that for me on the clay court it's much easier than on the other surfaces because we could see that I had much more time to organise my game.
"I just tried to stay focused and just believe in my chances and believe I can do it, play point after point, and then I just went all over the court."
