Serena Williams was thrashed 6-0 6-2 by Simona Halep at the WTA Finals in Singapore, equalling the worst defeat of her career.

The 18-time grand slam champion saw her 16-match winning streak brought to a shuddering halt by the Romanian in an incredible Red Group clash.

Williams, who won just nine points in the opening set, had not suffered such a heavy loss since she was beaten 6-1 6-1 by Joannette Kruger in Oklahoma City in 1998.

Williams gave no indication of the struggles that awaited her as she boomed down an ace to start the match between the players who were impressive in claiming straight-set victories in their opening Red Group matches.

That ace would be one of only nine points the 18-times grand slam champion would win in the opening set as her game fell apart, in the process giving Maria Sharapova hope of overhauling her to finish the season as world No 1.

Halep, serving supremely, needed only 12 minutes to race 4-0 ahead with Williams looking almost in shock at her routine failure to send simple groundstrokes over the net.

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.

Grew in confidence

The Romanian, runner-up at the French Open earlier this year, grew in confidence as Williams offered little hope of a recovery. She smashed an ace to take the set 6-0 at the third opportunity.

The last time Williams lost a set 6-0 was in Madrid last year at the hands of Anabel Medina Garrigues, but she bounced back to win the match and the tournament. That scenario never looked like repeating itself on Wednesday.

Jumping around in between points and screaming at herself in an attempt to snap out of the hole, she finally got herself on the board in the ninth game as she held serve to make it 2-1.

She then rallied on the impressive Halep serve to force a break point but the 23-year-old Romanian snuffed it out with another ace and went on to make the key hold for 3-1.

Halep stepped away from her traditional counter-punch game and went on the front foot, landing attacking ground strokes to move Williams around the court as she moved 5-2 ahead with the American despondent.

She claimed the famous win on her first match point and commented: "The biggest match of my life. I'm really happy.

"I think it wasn't her best day today. She missed a few important balls in the first set. But I think the second set was closer. I just wanted to fight for every point and to be focused for every point."

Williams added: "It was actually embarrassing I think describes the way I played. Yeah, very embarrassing. I've seen her play a lot. Like I said, she's never played like this before. My serve was at best in the 10 and under division in juniors."

In Wednesday's other singles match, Eugenie Bouchard suffered her second defeat of the group stages as she was beaten 6-1 6-3 by Ana Ivanovic.

Halep tops the Red Group with two wins from two while Ivanovic and Williams hold a record of 1-1 and Bouchard has two losses.