Britain's Heather Watson forced Maria Sharapova to battle for over three hours to reach the third round of the Toray Pan Pacific Open.
The 20-year-old took the first set on a tie-break and caused problems for her illustrious opponent throughout as she sought to follow up her upset of Sabine Lisicki on Monday.
But the world number two eventually prevailed 6-7 (7/9) 6-3 6-4 in three hours and 10 minutes.
Fourth seed Petra Kvitova was the biggest casualty of the day after she was beaten in straight sets (6-4 6-4) by Petra Martic.
World number one Victoria Azarenka had no such problems in booking her place in the last 16. The reigning Australian Open champion dismissed Austria's Tamira Paszek 6-1 6-1 to set up a clash with Roberta Vinci, who also had few troubles in her routine 6-3 6-1 victory over Pauline Parmentier.
Third seed Agnieszka Radwanska and her younger sister Urszula saw off the challenges of two former world number ones.
Agnieszka comfortably dispatched Jelena Jankovic 6-2 7-5 after Urszula had earlier upset former French Open champion and 11th seed Ana Ivanovic.
Fifth seed Angelique Kerber eased past wild card Ayumi Morita 6-3 6-4 and Sara Errani, seeded sixth, was a 6-3 7-6 (7/1) winner over Swedish qualifier Johanna Larsson.
Li Na and Samantha Stosur, seeded seventh and eighth, were both taken to three sets before prevailing against Anastasia Pavlyuchenkova and Francesca Schiavone respectively.
There were also wins for Marion Bartoli, Caroline Wozniacki, Dominika Cibulkova, Lucie Safarova, Nadia Petrova and Jamie Hampton.