German Kerber, the top seed, cruised by Japanese qualifier Risa Ozaki 6-2 6-2 while Czech second seed Pliskova beat 15th-seeded compatriot Barbora Strycova 6-1 6-4.
Kerber was barely tested in her 62-minute match during which she displayed a great arsenal of shots and was all over her opponents' serve.
She moves on to face the winner of Monday's round-of-16 clash between Venus Williams and Russian seventh seed Svetlana Kuznetsova.
Pliskova cruised through her first set in 22 minutes and looked ready to wrap up a quick victory when she went ahead 5-2 in the second but Strycova found life late in the match breaking serve then holding at love.
"Everything was fine, going well, until 6-1, 5-2," Pliskova, who fought off three break points in the final game, told the WTA.
I was happy to close it because the end was tricky - the end was a little bit shaky.
Sixth seed Muguruza took a medical timeout after dropping the first set 7-6(1) to Wozniacki after feeling dizzy and decided she was unable to continue against the Dane.
"I started normal, and then like at 3-3 or something like this I started to feel headache, pain in my stomach," Muguruza said. "And then it kind of went more and more during the match.
"When I was getting to the 5-4, something like this, I started to feel a little bit dizzy. I think it was the heat? The heat affected me suddenly like that."
With the win, Wozniacki advanced to sixth quarter-final in seven 2017 events.
In other women's action, former Czech world number five Lucie Safarova beat Slovakian fourth seed Dominika Cibulkova 7-6(5) 6-1 to reach the last eight.
For unseeded Safarova, who is on the comeback trail from a bacterial infection that hampered her for parts of the past two seasons, the win marked her first top-10 victory since 2015.
Read the original article on Eurosport: Kerber, Pliskova reach Miami quarter-finals, Muguruza retires