Serena Williams of the US hits a return to Simona Halep of Romania during their WTA Finals singles tennis match at the Singapore Indoor Stadium October 22, 2014. [Photo/Agencies]

SINGAPORE - Serena Williams was routed 6-0, 6-2 by Simona Halep at the WTA Finals on Wednesday in one of the most one-sided losses 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 just 16.

"Embarrassed is the way I would describe how I played," Williams said. "Definitely embarrassing."

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

The 6-0 first set was the first time since the 2008 championships that Williams had lost a set by that score, against sister Venus.

"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."

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

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.

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 was surprised by Halep's performance, saying "she has never played like this before."

"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.