Hurkacz beat Jannik Sinner 7-6(4), 6-4 in the men's final and says the win is a massive confidence boost.
World number one Ash Barty successfully defended her Miami Open title, beating Canadian Bianca Andreescu 6-3 4-0.
Barty returns to the final after beating No. 5-seeded Elina Svitolina 6-3, 6-3.
The Russian will face either Dusan Lajovic of Serbia or American Frances Tiafoe for a place in the quarterfinals of the prestigious ATP Masters and WTA hardcourt tournament.
In her first match since winning the Australian Open, women's world number two Naomi Osaka beat Ajla Tomljanovic 7-6 (7/3), 6-4.
After dominating the first set tiebreak, Barty broke the big-serving Pliskova in a 12-minute game to open the second set.
Failing to win his 850th match, the top seed Serb lost to 22nd-seeded Roberto Bautista Agut 1-6, 7-5, 6-3.
Williams withdrew with a left knee injury, tournament officials said. It is the second consecutive WTA event in which the American has been unable to continue, after her retirement from last week's tournament in Indian Wells with a viral illness.
Twenty-times grand slam champion Roger Federer suffers a shock 3-6 6-3 7-6(4) defeat by Australian Thanasi Kokkinakis in the Miami Open second round on Saturday.
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