Spaniard Muguruza dispatched Lucie Safarova 6-3 7-6 (4) before Germany's Kerber overcame an error-strewn and unwell Petra Kvitova 6-2 7-6 (3) in a battle of the lefties in another White Group encounter.
The 22-year-old Muguruza served confidently, raining down 10 aces, and controlled the baseline exchanges with aggressive strokes on the slower indoor hardcourt at the Singapore National Stadium.
The world number three from Spain has been in strong form since she lost to Safarova in the quarter-finals of the French Open,
"I think I'm making better choices...wait for the perfect ball to really attack," Muguruza told reporters.
"You have to be aggressive, but in a good way, making good choices."
Wimbledon runner-up Muguruza broke French Open finalist Safarova in the third game of the first set and pressurised her serve throughout a one-sided opening salvo.
Safarova broke at the start of the second set but the Spaniard, who won the biggest title of her career at the China Open earlier this month, broke back in the sixth game as she regained her rhythm.
The Venezuelan-born Muguruza broke the world number nine to love in the sixth game thanks to her heavy groundstrokes and dominated the tiebreak from the start.
REVENGE WIN
It was a revenge win after she and compatriot Carla Suarez Navarro lost to Safarova and Bethanie Mattek-Sands in their doubles opener on Sunday.
The day's second singles lacked the same quality with twice Wimbledon champion Kvitova all at sea, offering up 44 unforced errors.
A lethargic Kvitova, diagnosed with mononucleosis in August, rallied at the start of the second set to go 3-0 up, only to lose the momentum as world number seven Kerber raced 5-4 ahead with a booming forehand down the line.
The German showed nerves as she faltered and lost her serve when attempting to close out the match but Kvitova wasted set points at 6-5 as Kerber forced a tiebreak.
A wild service return then handed Kerber a winning start to the eight-player championships.
"I feel weird," said world number five Kvitova of her ailment. "It's really tough to describe.
"I couldn't really do what I wanted. I felt I wasn't able to stay in the kind of good rallying that she played."
(Editing by Tony Jimenez)
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