Williams has looked in ominous form so far at Melbourne Park, dropping just nine games, as she seeks one more Grand Slam title to put her alongside Margaret Court's record 24. She started positively against Venus, though, and broke her twice to take the first set.
Pliskova made only three unforced errors during the 60-minute match in which she dominated Muguruza in their baseline duels.
Williams will play the victor of the match between her older sister Venus and top seed Simona Halep for a place in the quarter-finals.
Naomi Osaka fought back from a set down for the second consecutive match to reach the quarter-finals as she bids to become the tenth woman to win the US Open and Australian Open back-to-back (Serena Williams was the last to do so in 2014-2015). "I'm supposed to be good at starting matches", Osaka said.
Osaka, who faces Latvian 13th seed Anastasija Sevastova next, said she found it hard to settle veteran world No. 27 Hsieh's dizzying mix of drop shots and slices.
The enthusiastic and aggressive Hsieh lost her way temporarily as Osaka reeled off four successive games with the end of the set looming, having been broken in the sixth game. Notably, she had to overcome a similar situation in her third-round match as she had stormed back after being a set down against Su-Wei Hsieh.
"I get that I'm tan and I would stand out a little bit in Japan", Osaka said.
"I personally think she was making it a bigger deal than it had to be", Osaka said. "I'm not even dealing with that right now".
Despite being the reigning US Open champion and one of the world's best players, Osaka has admitted she hasn't really felt the love from the Melbourne public. "I shouldn't be sad, I'm playing against a really great player, so I should just enjoy it and try and do the best I can".
Hsieh won 7 of the following 8 games to lead 7-5 4-1.
Hsieh also benefited from Osaka's numerous backhand errors to break serve and lead 4-2. "I was doing pretty well until the second set".
"Hopefully learn from that moment", she said later.
Osaka may not have been on top gear but she found ways to get the job done against Sevastova.