Fresh from her silver medal at the Rio Olympics, second seed and Australian Open champion Kerber was relatively comfortable against Kristina Mladenovic in Cincinnati.
Halep cruised, while that duo were joined in the third round by Garbine Muguruza on a largely good day for the seeds.
KERBER, HALEP AND MUGURUZA WIN THROUGH
Mladenovic had won through the first round earlier in the day, and she proved no match for 2012 runner-up Kerber in a 6-0 7-5 defeat.
Last year's runner-up, Halep, was even more comfortable, thrashing German Annika Beck 6-3 6-1 in just 55 minutes.
Muguruza claimed a 7-6 (7-4) 6-2 win over American Coco Vandeweghe, while Agnieszka Radwanska crushed Andrea Petkovic 6-0 6-1.
BENCIC, STOSUR BEATEN
On her return from injury, Swiss 13th seed Belinda Bencic suffered a 7-5 6-2 loss to qualifier Timea Babos.
Australia's Samantha Stosur also went down, losing to Barbora Strycova 7-6 (13-11) 6-3, while Elina Svitolina was comprehensively beaten by Daria Gavrilova 6-2 6-2.
PETKOVIC DOWNED, DOI TAKES CHANCE
Petkovic needed two hours and 29 minutes to win her first-round match, and returned to the court just hours later to be beaten by Radwanska.
Misaki Doi, the lucky loser who replaced the injured Serena Williams in the draw, made the most of her chance with a 2-6 6-3 6-4 win over wildcard Christina McHale.
BIG GUNS BACK UP
The WTA Premier event has been hit by rain, leading to numerous delays to start the tournament. Kerber takes on Strycova on Thursday, while Halep meets Gavrilova and Radwanska faces Johanna Konta in a blockbuster clash.