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Australian Open champion and second seed Osaka benefited from a walkover after qualifier Nina Stojanovic withdrew ahead of Sunday's clash due to a right thigh injury.
Elise Mertens will face Osaka following the 16th seed's 6-2 0-6 6-2 win against Anett Kontaveit.
Two-time grand slam winner Muguruza rallied past Russian wildcard Anna Kalinskaya at the WTA Premier event.
Spanish star Muguruza dug deep to earn her tour-leading 20th win of the season.
Former world number one Muguruza – the 12th seed – prevailed 4-6 6-3 6-4 against Kalinskaya, having trailed 3-0 in the final set.
"I brought my fighting spirit and I could match the level of fighting against Anna, and very happy to go through these tough matches," Muguruza said following two hours, 41 minutes on court.
"Doesn't matter if you play pretty or you play ugly, but you stay strong and you go through these rounds. That gives me an opportunity to feel better and to have another chance."
Muguruza – who has the most final appearances this season with three, having won the Dubai Tennis Championships and finished runner-up at the Qatar Open and Yarra Valley Classic – will next face eighth seed Bianca Andreescu.
Former US Open champion Andreescu overcame fellow seed Amanda Anisimova 7-6 (7-4) 6-7 (2-7) 6-4.
Meanwhile, fourth seed Kenin was sent packing as sixth seed Pliskova was tamed by nemesis Jessica Pegula in Miami.
Stunned in the second round of the Australian Open – in her title defence – Kenin then crashed out of the Phillip Island Trophy at the first hurdle.
Kenin rallied to beat Andrea Petkovic first-up in Miami, but the 22-year-old was beaten 6-4 4-6 6-4 by 27th seed Ons Jabeur.
Jabeur and Sara Sorribes Tormo – who shocked 21st seed Elena Rybakina 6-1 3-6 6-2 – will meet for a spot in the quarter-finals.
PLISKOVA FALLS TO PEGULA… AGAIN
For the third time this past month, 2019 Miami Open runner-up Pliskova went down 6-1 4-6 6-4 to 29th seed Pegula.
Australian Open quarter-finalist Pegula conquered Pliskova in Doha and Dubai before repeating the feat in Miami on Sunday.
"I played her a lot the last three weeks, three tournaments," Pegula said. "I kind of expected her to play well there. Even when I was up I still felt like she could easily come back just because she plays so aggressive and she serves well."
After reeling off the final four games, Pegula set up a last-16 showdown with 23rd seed Maria Sakkari, who dismantled qualifier Liudmila Samsonova 6-0 6-1.