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 semi-final berth, needing to win just one game against Ivanovic, while Bouchard has only a very slim mathematical chance of progressing.

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