Serena Williams admitted her fitness was "nowhere near 100 percent" as her WTA Finals hopes receded after one of the heaviest defeats of her career on Wednesday.

Williams came into the eight-player season finale with a knee injury and she said she served like a 10-year-old in a stunning 6-0, 6-2 loss to Simona Halep.

"My forehand was off today again -- I guess it went on an early vacation," Williams said. "Lord knows my serve was as well.

"My serve, I don't even... I don't know. My serve was at best in the 10 and under division in juniors.

"Yeah, it was actually embarrassing I think describes the way I played. Yeah, very embarrassing."

The world number one served seven double-faults, won just 29 percent of points on her second serve and was broken five times in what was her biggest loss since 1998.

Williams ground out a 6-4, 6-4 win over Ana Ivanovic in her opening match and she will play Eugenie Bouchard, whom she beat in three sets last year in Cincinnati, in her final round robin match in Red Group.

But the 18-time Grand Slam-winner, who withdrew from this month's China Open with her left knee problem, said she was far from full fitness.

"Oh, God no. I'm definitely not 100 percent okay. I'm just here playing, but I'm not nowhere near 100," she said, when asked about her injury.

"That has nothing to do with today's match. I think Simona played really well and the best match of her career.

"To be quite frankly honest, I'm looking forward to our next meeting because she is making me going to go home and work hard and particularly train for her."

Despite her problems, Williams can take heart from the fact that since the round robin format returned in 2003, four players have lost a group match and then gone on to win the tournament.

"It's odd. I feel like once you lose you should be out of event, you know. But I'm still hanging in there, so we'll see what happens," she said.

"I played when it was an elimination. At one point when I was 16 it was by elimination. I did like that format.

"This format is fun because you can lose and keep winning. But I'm in a situation where I'm going to have to win my matches because I didn't win many games today at all."