SINGAPORE (AP) -- Serena Williams did not hesitate when asked to describe her performance Wednesday, when she took her heaviest defeat since she was 16 years old.

''Embarrassing describes the way I played today,'' Williams said. ''Very embarrassing.''

Williams was routed 6-0, 6-2 by Simona Halep at the WTA Finals in what was arguably the worst performance of the 18-time Grand Slam champion's career.

The last time Williams managed to win just two games in a WTA Tour or Grand Slam match was in 1998 when she was a newcomer to the tour, known best as Venus' little sister.

The loss in the round-robin match provided a shuddering halt to Williams' 16-match winning streak at the year-end championships, where she is the two-time defending champion.

''My forehand was off today,'' Williams said. ''It's obviously gone on an early vacation. My serve was at best at the 10-and-under division of juniors.

''You got to step up to the plate. She did and I didn't.''

The fourth-ranked Halep had never before beaten a player ranked in the top 3, and in her two matches against Williams last year, she only won a total of seven games.

For a player who was ranked No. 47 two years ago, Wednesday's win over No. 1 Williams represented a new career high.

''It was my best match of my life,'' Halep said. ''I played unbelievable shots. It means a lot this match. At an important time I played really well, and after today my confidence will be more high.''

Halep, runner-up at this year's French Open, beat Eugenie Bouchard in her opening match and moves to 2-0 in the Red Group. Williams slips to 1-1 and will face Bouchard next.

Ana Ivanovic beat Bouchard in Wednesday's other game, winning 6-1, 6-3. Ivanovic is 1-1 and Bouchard fell to 0-2. Going into the last round-robin matches in the Red Group, Halep is all but certain of a semifinal berth, needing to win just one game against Ivanovic, while Bouchard has only a very slim mathematical chance of progressing.

Williams' defeat jeopardized her chances of finishing the year as the top-ranked player. Had she won Wednesday, the only way she could have lost the No. 1 spot is if Maria Sharapova won the title.

Now, just reaching the final may be enough for the Russian, depending on the result of Williams' match against Bouchard on Thursday.

Halep has been careful not to raise her sights too high at this tournament. She remained cautious even after two dominant victories.

''I have confidence I can get to the semifinals,'' Halep said. ''First I have to forget this day, but it will be hard to forget. I am so happy.

''I will party with my friends and family, and for sure I will enjoy this victory, because it means a lot.''

Williams served six double-faults in the opening set and had 36 unforced errors - three times Halep's total. But the key to the result was the performance on break points. Both players had six: Williams took none of them while Halep converted five.

Williams praised Halep's performance, saying ''she has never played like this before'', while also vowing to make amends next time.

''To be frankly honest, I am looking forward to our next meeting because she is going to make me go home, train hard and particularly train for her,'' Williams said.

In Thursday's matches in the White Group, Carolina Wozniacki meets Agnieszka Radwanska in a clash of two players who won their opening matches, while Sharapova faces Petra Kvitova.