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Briton Boulter, ranked 262 in the world, triumphed 6-4 6-3 on Wednesday to impressively eliminate sixth seed Bouzkova, who was only able to convert one of her seven break-point opportunities.
Seventh seed Mladenovic, meanwhile, fought hard for a 6-2 5-7 6-2 triumph in a battle lasting two hours and 15 minutes against Japan's Kurumi Nara.
Nara converted all four of her break points but ultimately could not pull off the upset.
There were more comfortable days on court for Alison Van Uytvanck – who thrashed Marina Melnikova 6-1 6-1 – and fourth seed Zhang Shuai, a 6-3 6-4 victor over Arina Rodionova.
American Caty McNally continued her progress with an easy 6-0 6-3 win over Nao Hibino to book what looks like an exciting last-16 tie against Mladenovic.
Second seed Alison Riske will play Xiyu Wang in the last match of the day, while top seed Johanna Konta continues her campaign against Kateryna Kozlova on Thursday.