When I wrote my preview of the WTA Finals last weekend, I picked Simona Halep, Flavia Pennetta, Petra Kvitova, and Angelique Kerber to make the semifinals.

I thought Halep would play well in her return to Singapore, where last year she reached the final and recorded her only win over Serena Williams. Instead, the top seed seemed constrained by her own expectations, and exhausted after nearly a month off.

I thought Pennetta would play well because she would be loose and relaxed in the final event of her career. Instead, she came out looking jet-lagged and rusty in her opener against Halep, and while she got better as she went, she couldn’t recover from that blowout defeat.

I thought Kerber would take the steady, gritty route through, as her opponents went up and down, and would win the sets necessary to qualify. But again, she showed that when the tournaments get bigger, her game doesn't get bigger with them.

Now the round robins are over and the semifinals are here; of my picks, only Kvitova remains. Which makes a perverse kind of sense. Leave it to the ever-unpredictable Petra to defy the odds and survive my prognostications.

But two of the other semifinalists, Maria Sharapova and Garbine Muguruza, did more than survive. By dominating their groups with 3-0 records, they defied any and all doubts that I had about them coming in. Along the way, they’ve also set themselves up as early Grand Slam sleepers in 2016.

Before the tournament, I believed that Sharapova, who hadn’t played a full match since Wimbledon, would have at least a little rust to shake off. And she did, but it only lasted for a set against Agnieszka Radwanska; after that, the 28-year-old Russian was as steely and hungry for success as she had been when she was 18.

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Singapore Semifinal Previews

Singapore Semifinal Previews

As for Muguruza, I thought that a No. 2 seeding at her first WTA Finals might be a little too much, too soon. She has obviously been hot this fall, but her all-in attacking game still carries a substantial amount of risk. Yet even when she wasn’t sharp, Muguruza belied her 22 years and found a way to win anyway. Her competitive skills seemed to improve with each set; by the time she had snuck past Kvitova 7-5 in the third on Thursday, she was playing like an opportunistic veteran. In some ways, Halep got out of Singapore just in time; as she walked away, she might have heard Muguruza’s loud footsteps stomping up behind her.

If form holds, Muguruza will meet Sharapova in a season-ending final-round shootout. But will form hold? Here’s a quick look ahead at Saturday’s semifinals.

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At this point, the year really does seem like one long blur: These two have already played four times in 2015, but I can only recall their see-saw, three-set Wimbledon semifinal. It’s not as if the others weren’t memorable, though; two of them went the distance, including the most recent, in the Beijing semis earlier this month.

Whatever the scores and the circumstances of those four meetings, the result was the same: A Muguruza victory. Put them all together and the Spaniard has single-handedly kept the Pole from having a much better season than she did. Is there any reason to think that Radwanska can turn the tide this time? Muguruza is 3-0 in Singapore; Radwanska is just 1-2, and she had to rely on a few things breaking her way to make the semis. But in her win over Halep on Thursday, Aga had all of the various elements of her game in good order, while Muguruza, who is also in the doubles draw this week, and who will be playing without a day’s rest, looked a little gassed against Kvitova. Winner: Radwanska

Singapore Semifinal Previews

Singapore Semifinal Previews

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As with the first semi, this one pits a 3-0 group winner in Sharapova, against a 1-2, there-by-the-grace-of-God survivor in Kvitova. Unlike that match, though, the player with the better record also appears to be the player who is in better physical condition for this one.

Sharapova, who leads her head to head with Kvitova 6-3, has grown stronger as the week has progressed, and she’ll be playing after a day of rest. Kvitova, meanwhile, has been fighting a mono diagnosis and has looked sluggish at times during her first three matches, two of which she lost. Petra beat Maria at this event last year, but coming back one day after losing a three-setter will be a big ask. Winner: Sharapova