British number one Johanna Konta is out of the Pan Pacific Open after a 7-5 7-6 (7-5) loss to Barbora Strycova.
Konta was making her first appearance since a shock US Open first-round defeat to unseeded Serb Aleksandra Krunic and made a strong start as she opened up a 3-0 lead in the first set.
However, the Czech world number 25 fought back to take the opener and saved two set points before going on to secure the victory via a tie-break.
"I'm very emotional and very happy that I won against Johanna," said Strycova.
"I tried to focus on every point. She's such a tough opponent to play, she doesn't give you anything for free, you have to work for it."
Strycova will now take on Anastasia Pavlyuchenkova in the quarter-finals.
Elsewhere, Caroline Garcia and Dominika Cibulkova secured straight-set victories over Kurumi Nara and Katerina Siniakova respectively while Caroline Wozniacki overcame Shelby Rogers 2-1.
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