In the late match on Wednesday, Ukranian Elina Svitolina meets Caroline Garcia of France.
Svitolina was also making her debut in Singapore after a stellar campaign in which she won five titles, but the World No. 4 was overwhelmed by Wozniacki, who won 10 games in a row to turn a close match into a rout.
"I feel great that the first tournament where I'm No.1, I won the first match", Halep said.
The second set continued in favour of Halep as the Madrid Open champion in the third game of the second set before she fought off three break points on her serve to consolidate her advantage and confidently closed out the match to secure victory in her maiden match as the world No 1.
Halep stressed she was keen to learn from both that defeat and a win over the Frenchwoman she enjoyed in Canada two months ago ahead of the showdown in Singapore as the pair opened Red Group play with a high-class baseline tussle. "But this is not the most important thing for me at this time". She surrendered her serve when she double-faulted in the sixth game.
Wozniacki is now all but guaranteed a place in the semi-finals, while Halep will very likely have to beat Svitolina to reach the last four and keep alive her hopes of finishing the year at the top of the rankings. "I'll try to forget this day and try to play my next game as if nothing had happened", said the Ukrainian, dépitée.