The Romanian debutante said she had not even dreamt of such an upset after she won 6-0, 6-2 in Singapore, handing the 18-time grand slam-winner her heaviest defeat since 1998.
"No, I didn't," she said, when asked if she had ever imagined such a big victory against Serena, the world number one and two-time defending champion.
"But, you know, I think it she wasn’t at her best day today [Wednesday]. She missed a few important balls in the first set. But I think the second set was closer."
The French Open finalist also impressed in her opening 6-2, 6-3 win over Eugenie Bouchard at the eight-player season finale, and said she was now confident about reaching the semi-finals on Saturday.
"I can say I have confidence that I can go into the semi-finals. I have a chance now," said the 23-year-old world number four, calling it the best win of her career.”
While Serena and Maria Sharapova are the only two players to win the elite event on debut, Halep said Wednesday's victory felt like a breakthrough.
"It's the first time I've beaten Serena. She's number one. It's like a dream to beat one of the [Williams] sisters because they were – they are – very good players," she said.
Meanwhile, a sombre and downbeat Serena said her performance was best described as ‘embarrassing’.
“Yeah, my forehand was off today [Wednesday] again, she said. “I guess it went on an early vacation. Yeah, it was actually embarrassing; I think that describes [perfectly] the way I played.”
Meanwhile, the second match of the day saw Ana Ivanovic recover from her opening loss to race to a 6-1, 6-3 victory over an out-of-sorts Eugenie Bouchard on Wednesday, putting the world number seven back into the mix for a WTA Finals last-four berth.
"This is the beauty of the round-robin system — that I still had a chance,” said Ivanovic.
On the other hand, an out-of-contention Bouchard will now be looking for a consolation win when she takes on Serena in the final round-robin match on Thursday.
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