Kerber squandered a 4-0 lead in losing the first set. Pliskova held her serve twice in the second, Kerber only once, and that decided the match.
Kerber is ranked No. 9 in the world, while Pliskova is No. 64.
In another upset, Karin Knapp of Italy outlasted seventh-seeded Caroline Garcia of France, 6-7 (9), 6-4, 6-4 to reach her first semifinal in 10 months.