Halep, whose five WTA titles this year was second-most on the tour (Serena Williams had 11), defeated seventh-seeded Alize Cornet 6-4, 6-4 with two breaks in the first set and four in the seesaw second.
Still it was enough to move her to 2-0 in Group A round-robin play. Cornet drops to 1-1. Anastasia Pavlyuchenkova, the sixth-seed, is also 1-1 after she beat Elina Svitolina 6-2, 6-4.
Pavlyuchenkova won more than half (36-of-67) of the points off Svitolina's serve and finished with five breaks.
Svitolina was substituting for third-seeded Maria Kirilenko, who withdrew during Tuesday's match against Cornet with a knee injury.
No. 2-seeded Ana Ivanovic and fourth-seeded Samantha Stosur, who play in Thursday's first match, are both 1-0 in Group B. Ivanovic defeated Tsvetana Pironkova 6-0, 6-2 on Tuesday and Stosur was a 6-3, 6-3 winner over fifth-seeded Elena Vesnina Wednesday.
Stosur dropped her serve just once Wednesday while winning two-thirds of the points on serve. She also had four breaks.
Vesnina will take on the No. 8-seeded Pironkova Thursday with the loser virtually eliminated. They are both 0-1 in Group B play.
The third match Thursday sees Halep going for the Group A sweep against Svitolina.