Wozniacki had started her fifth appearance at the season-ending tournament with a 6-2 6-0 victory over Ukraine's Elina Svitolina and Romania's Halep appeared equally as powerless against the 27-year-old former number one.
The Dane's victory margin assured her of a last-four berth after the loss of four games in two matches while Halep lives to fight another day with her opening straight-sets win over Caroline Garcia giving her a 1-1 record in the Red Group.
Frenchwoman Garcia recovered from her opening loss to rally past Svitolina with an epic 6-7(7) 6-3 7-5 victory later in the day, a result that leaves the other three women in the group all with a chance of joining Wozniacki in the semis.
Wozniacki has been the form player on the WTA Tour this season, winning 58 matches, reaching seven finals and claiming one tournament victory in Tokyo last month.
The rankings show Halep as the world leader and Wozniacki in sixth but the Romanian was utterly outclassed as her opponent raced through the first set in 23 minutes on the back of a 94 percent first serve success rate.
"I think I played really well today and stayed aggressive throughout. I didn't expect to be leading by that much in the first set and I started to think 'what's happening? Am I really playing that well?'," Wozniacki said in a courtside interview.
Halep had been as much a spectator as the stunned crowd packed into the Singapore Indoor Stadium and although she tried to rouse herself after getting to 30-30 in the first game of the second set, Wozniacki held firm for a 1-0 lead.
FLEETING RESPITE
The Dane had won 17 games in a row across her last two matches before Halep finally got on the scoreboard after 34 minutes when she held to love.
The respite was fleeting, however, as Wozniacki continued to punish Halep's scrappy forehand to break for a 3-1 lead and an ace snuffed out the only break point the Dane faced all match as she moved within two games of victory.
Another forehand error handed Wozniacki a match point after a little more than an hour, which she duly converted with a forehand winner.
"I just stayed positive, took the ball early and went for my shots," the Dane added. "It's a great court for me as I was putting spin on the ball to get in position for my shots and wait for the right opportunities to hit my winners."
Garcia was the more aggressive of two well-matched players, even forging a pair of set points in the opening tiebreak, but sloppy forehand errors handed Svitolina the initiative as she edged the tightest of first sets.
Svitolina broke early in the second to assume control until Garcia threw caution to the wind, after shedding a few tears following a pep-talk from her father and coach Louis-Paul, and reeled off five of the next six games to level the match.
The decider followed a similar pattern to the second set with Svitolina keeping her nose ahead until Garcia fought back through a barrage of winners to reel off the last four games and seal her first WTA Finals victory.
Halep will hope to advance with a win over Svitolina on Friday, while Garcia faces the unenviable task of taking on Wozniacki with the world number eight needing a victory to stand any chance of reaching the last four.
(Editing by Ed Osmond)
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