Unseeded Kazakh Zarina Diyas caused an upset at the Citi Open, shocking Lesia Tsurenko for a place in the quarterfinals.

After eliminating Wimbledon sensation Coco Gauff in the round of 32, Diyas upstaged fifth seed Tsurenko 6-4 6-4 on Thursday.

Diyas broke Tsurenko five times throughout the one hour, 53-minute contest in Washington, where both players required medical timeouts in muggy conditions.

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Diyas – who lost to Tsurenko in Eastbourne in June – will next face Camila Giorgi for a spot in the semifinals of the WTA International event.

Giorgi dropped the first set, but the Italian rallied as she overcame Rebecca Peterson 3-6 6-3 6-4.

Fourth seed Hsieh Su-wei recovered from a slow start to beat Varvara Gracheva 7-5 4-6 7-6 (8-6) in Thursday's final match.

Hsieh trailed 5-2 in the opening set but she reeled off five consecutive games to move ahead in the match before Gracheva levelled it up. However, the former was not to be denied in a third-set tie-break.

Awaiting Hsieh is American wildcard Caty McNally, who prevailed 6-3 1-6 6-3 against Christina McHale.

Vekic overcomes Azarenka in San Jose, Mertens exits

Victoria Azarenka was beaten by Donna Vekic at the Silicon Valley Classic, while two seeds made second-round exits on Thursday.

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Azarenka went down to fifth seed Vekic 6-4 6-3 at the WTA Premier event played on hard-courts in San Jose, California.

Both players created eight break-point opportunities, but Vekic made the most of hers, converting five compared to Azarenka's three.

The Croatian moved into the quarterfinals, where qualifier Kristie Ahn awaits. Ahn caused an upset with a 6-3 6-3 victory over Belgian third seed Elise Mertens.

Another seed, Danielle Collins, was also resoundingly beaten, suffering a 6-2 6-0 loss to Zheng Saisai. Zheng will meet Amanda Anisimova after the 17-year-old fourth seed cruised past Madison Brengle 6-2 6-2.