Serena Williams' hopes of winning the WTA finals in Singapore have receded after one of the heaviest defeats of her career.
Williams came into the eight-player season finale with a knee injury and said she served like a 10-year-old as she was taken apart 6-0 6-2 by the Romanian Simona Halep, the American's heaviest loss since 1998.
The world number one ground out a straight sets win over Serbia's Ana Ivanovic in her opening match, and Williams will play the Canadian Eugenie Bouchard in her final round robin match.
It's the first time the rapidly improving Halep has beaten a top-three player.
She made this year's French Open final but lost to Maria Sharapova.
The loss halted the double defending champion's winning streak at the season-ending event at 16 matches and leaves her needing to beat Eugenie Bouchard, who later lost 6-1 6-3 to Ana Ivanovic, to reach the last four from the Red Group.
"It was actually embarrassing I think describes the way I played. Yeah, very embarrassing," Williams told reporters, before heaping praise on Halep.
"I've seen her play a lot. Like I said, she's never played like this before."
The American, looking for a fifth WTA Finals title, also tossed in six double faults with the few second serves that did make it over getting rough treatment from Halep.
Heading into the final round of matches, all four players can still advance from the Red Group although Halep is virtually assured of qualifying after winning her first two matches while Williams and Ivanovic have a win and loss apiece.
Bouchard faces an almost impossible task to reach the semi-finals after two heavy straight sets defeats but is still mathematically in with a chance of advancing.