Serena Williams
Serena Williams displayed her frustration throughout defeat to Simona Halep in SingaporeGetty Images/MOHD FYROL

World number one Serena Williams crashed to the joint-heaviest defeat of her career as Simona Halep shocked the 18-time grand slam champion at the WTA Finals in Singapore.

Halep prevailed 6-0 6-2 in just 65 minutes to claim the second win of the round robin stage to all but book a place in the semi-final and leave her American opponent reeling.

Williams was broken five times by the number four seed and registered just two games to suffer her worst defeat since the 1998 6-1 6-1 loss to Joannette Kruger.

"My serve was at best in the 10-and-under division in juniors. It was actually embarrassing I think describes the way I played," Williams said.

"I think she played really well, to be honest. Personally I've never seen her play like this, ever. But obviously she had nothing to lose and went for a lot of shots. You know, I think she just played a really, really good game.

"I'm obviously frustrated and I wanted to do better. I wanted to win more than two games. I was trying for three.

"At one point I was like: 'If I can just get three games...' But that didn't work out for me. But it's OK. You know, things are going and going and I'm still going to come back and I'm going to be better and I'm going to keep trying to do better."

Halep, 23, meanwhile has all but secured a place in the last four in her first appearance at the season-ending tour finals after one of the biggest wins of her six-year professional career.

She was defeated in three sets by Maria Sharapova in the French Open final in June but Halep said victory over reigning WTA Finals champion Williams ranked above her feats at Roland Garros.

"It's the biggest match in my life, so I can't put into words how I feel right now. I feel very happy. I just want to enjoy it," said an elated Halep.

"It was always my dream to beat one of the Williams sisters but of course without the help of the crowd, I wouldn't have been able to play this way today.

"I don't think it was her best day today but I played really well. I was very aggressive and took the ball early, moved up the court very well, served very well. I've found my best tennis again. It's crazy."