The 26-year-old will ensure qualification to the semi-finals if she beats Elina Svitolina in a crunch match on Friday.
Wozniacki, who had already qualified from the Red Group after two wins, had lost just four games in five sets of tennis after taking the opener 6-0 against Garcia.
Even though her campaign ended in disappointment, Halep will still finish the year as world number one if Pliskova does not lift the trophy tomorrow. She admitted later: "Today I just felt 100 per cent that the court is not for me and I can not finish a point". "So it was a little bit tough to find my rhythm and also to make a victor". As she cranked up the victor count, she broke again, and served out her first ever win in Singapore, 7-5. "It means a lot (to win)".
The Ukrainian dictated from the start, breaking Halep's tentative serve straight away and winning the opening three games before claiming the first set in 31 minutes.
Granted, there was a "heat of the moment" element to Svitolina's comments, given that she was speaking in the immediate aftermath of a frustrating three-set loss to Caroline Garcia - a match she really should have won.
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Pliskova led 3-2 in the first set of the early match, but the already-eliminated Ostapenko won the next seven games for a 6-3, 3-0 lead.
Svitolina, victor of no fewer than five titles this year, showed her mettle with a quick break in the third set, and after conceding a break back, fired up her backhand to make two cross-court winners past the net-bound Garcia.
"It was kind of weird, actually, to be here". I wish I had no chance to qualify, but - yeah, okay. "Every time I go on court, I just want to be better than I have been two days ago or one day ago".
"I don't think it's ever easy when you play a world No. 1", Wozniacki said. "I must give her (Wozniacki) credit for the victory". "I think this court is great for the all-around player who can play both aggressive and defense."The sixth-seeded Wozniacki saved the one break point she faced in the fifth game of the second set". "I missed my chances".