The 27-year-old stunned Halep 6-0, 6-2 in just 63 minutes in Singapore to storm into the last four of the year-end tournament for the loss of just four games.
Wozniacki, a renowned counter-puncher, said she has enjoyed playing a more aggressive game in the round-robin format.
“Even if you lose the first match, you still have nothing to lose,” she told reporters. “It’s just a nice cushion to have. I think that kind of motivates me to start off strong and just go for it.
She added: “I just ran a lot of balls down and I played aggressive and I mixed up the pace. Everything I wanted to do was going my way.”
The Dane said she was well suited to the slower court of Singapore, where she was a semi-finalist in 2014.
“I think this court is great for the all-round player, who can play both aggressive and defence,” she said. “I just have been feeling good and am playing at a high level.”
It was a bitterly disappointing evening for Halep, who was impressive in her opening victory over Caroline Garcia.
“I made too many mistakes and she didn’t miss,” Halep said. “It was one of my bad days. During the points I missed too many and I wanted to over-hit the ball.”
The 26-year-old will ensure qualification to the semi-finals if she beats Elina Svitolina in a crunch match on Friday.
In the other match of the evening, world number eight Garcia kept her WTA Finals campaign alive with an epic three-set victory over Svitolina.
Down 5-3 and on the brink of elimination, Garcia dug deep and reeled off four straight games to prevail 6-7 (9/7), 6-3, 7-5 in two hours and 44 minutes. It was the pair’s second consecutive marathon following Svitolina’s victory earlier this month in Beijing.
Garcia turned the tide through her high-voltage groundstrokes, enjoying a sizeable 58-33 advantage in winners. It means Garcia, Svitolina and Halep will battle for the final semi-final position from the Red Group.
It was a stellar comeback from Garcia, who blew two chances to win the first set tie break. An emotional Garcia went back to her chair and started crying but regathered herself to eventually pull out a victory.
“On court I was obviously feeling nervous,” she said. “I’m very happy to be through with a win. It was an up-and-down match like the one we played in Beijing.”
Garcia still has a chance to top the group if she beats Wozniacki and Svitolina defeats Halep.
A disappointed Svitolina said it was a “terrible feeling” to have lost after being in a commanding position. “I missed my chances. I mean, my game was completely off,” she said.
“Unfortunately, I cannot go back in time.”
Svitolina will need to defeat Halep in straight sets coupled with a Wozniacki win to qualify.