Halep, now ranked No 2, defeated the unseeded Daria Kasatkina 6-2 6-1 to close in on the top spot held by Garbine Muguruza.
But Halep steeled herself and reeled off three successive games to become the 25th player in the history of the WTA rankings to sit at top spot.
Throughout 2017, Halep has come agonisingly close to becoming world number one, only to lose a match which would have resulted in her claiming the much sought-after position. Halep, thus, replaces Garbine Muguruza at the top of the rankings which will be updated on Monday.
Halep is yet to win a Grand Slam but reached her first major final at the French Open this year.
The former world No. 1 was forced to retire during her Beijing opener due to illness.
"Of course it's the best moment in my life, and I want just to keep it", Halep told reporters in Beijing. In the quarter-finals, she meets the victor between 11th-seeded Agnieszka Radwanska of Poland and Russian Daria Kasatkina.
Energy East pipeline cancelled
The Federal Court of Appeals in Vancouver is hearing arguments this week and next on a slew of consolidated challenges. The pipeline, which drew pointed opposition in Quebec, was undergoing an environmental review in that province.
In New York the Russian was able to dominate Halep's serve but this time the Romanian was broken just once, in her opening game.
"She was hitting the ball consistently, not making a lot of unforced errors, her service percentage was quite high".
French Open champion Jelena Ostapenko of Latvia qualified for the season-ending WTA Finals in Singapore with a 6-3 7-5 win over Australia's Samantha Stosur in round two.
Third seed Elina Svitolina defeated in-form Australian Ashleigh Barty 6-4 6-2.
The last Premier Mandatory event of the season has turned into a place of retribution Halep who cast out some of her recent demons.