Simona Halep hits a return to Serena Williams during their WTA Finals singles match at the Singapore Indoor Stadium on Wednesday.

AFP/Singapore

Simona Halep stunned Serena Williams 6-0, 6-2 at the WTA Finals on Wednesday, dealing the world number one her heaviest defeat in 16 years and denting her chances of a third straight year-end title.

The diminutive Romanian exploited some horrendous serving by Williams to clinch her first win against the world number one and go top of Red Group in the eight-player season finale in Singapore.

The 23-year-old Halep was the aggressor all the way, leaving Williams, 10 years her senior and the oldest player in Singapore, frustrated with her pinpoint placement and fearless passing shots.

"Yes, I think it was my best match in my life," said the world number four, the first Romanian to qualify for the tournament since 1998.

"I'm a little bit surprised, but I knew before the match that I have nothing to lose, just play. She's the best player in the world, she's number one.

"It's the last tournament of this year. I had injuries. I had a tough period. Now I'm really happy and I really believe in myself."

Halep, who had never beaten a top-three player before, saved six break points in the second set as she resisted the 18-time Grand Slam-winner's efforts to save the match in front of a shocked crowd.

Halep now has a 7-4 record against top 10 opponents this year and before Wednesday's win, she was 0-4 against current world number ones.

Williams is bidding to become the first player to win the season finale three times in a row since Monica Seles in 1992, and is also hoping to secure the year-end world number one title.

She next faces Canada's Eugenie Bouchard, who has also lost to Halep, in her final group match.

The most formidable serve in women's tennis deserted Williams, who was broken five times, notched seven double faults and made only 59% of her first serves, to Halep's 64%.

"My serve was at best in the 10 and under division in juniors," Williams quipped after the match.

"Yeah, it was actually embarrassing," she added. "Yeah, very embarrassing."

It was Williams' worst result since 1998 when she lost 6-1, 6-1 to Joannette Kruger in Oklahoma City.

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