Madison Keys suffered an upset at the Charleston Open, with Anna Kalinskaya winning in straight sets to book a place in the quarter-final.

The Australian Open champion struggled to find her groove and had no answers as Kalinskaya ran out a 6-2 6-4 winner in just 80 minutes.

It was Kalinskaya’s first top-10 victory in 10 months as she reached her second quarter-final of the season, where she will face Sofia Kenin.

"This one is special. I needed this for my confidence," Kalinskaya said.

"I had an unfortunate start to the year, but it's nice to be back and playing at such a high level. I think the key tonight was to change a little bit the rhythm. I did a few drop shots, especially in the first set.

"I didn't want to go into very long rallies, because she's a great player and she has unbelievable strong shots, so I tried to step in on the serve, be aggressive as much as I could, and be very consistent ... to keep the level, and I think that was the key."

Quarterfinals booked

Anna Kalinskaya books her spot in the quarterfinals defeating the No. 2 seed Keys 6-2, 6-4.#CharlestonOpen pic.twitter.com/EHn74QzYa3
— wta (@WTA) April 3, 2025

Elsewhere, Jessica Pegula booked her place in the last eight with a comfortable 6-3 6-2 victory over Ajla Tomljanovic.

The pair played out a dramatic three-set encounter when they met in Austin last month, but Pegula wrestled control on Thursday, despite losing her serve in the first game and missing the chance to convert the first five set-point opportunities she had.

She will be playing her fifth quarter-final of the season when she faces reigning champion Danielle Collins, who beat Jelena Ostapenko to continue her title defence.

Collins had to rally from a break down in both sets but recorded a 7-5 6-3 victory over the 11th seed for her eighth consecutive win in Charleston.

Zheng Qinwen had to do things the hard way to make it to the quarter-final, and earn her 13th straight clay-court win.

She also had to come from a break down in the deciding set against Elise Mertens to win 6-3 3-6 6-3, with Ekaterina Alexandrova next up.

Emma Navarro will play in her first-ever quarter-final at the Charleston Open on her seventh attempt, in an all-American encounter with Amanda Anisimova.

Navarro had to come from a set down to do so though, beating another compatriot in Ashlyn Krueger 4-6 6-4 6-2.

All GRIT

Emma Navarro rallies from a set down to defeat Krueger and sets up a meeting with Anisimova in the quarterfinals!#CharlestonOpen pic.twitter.com/JsSPA3NOm4
— wta (@WTA) April 3, 2025

Data Debrief: Keys comes unstuck

Keys has shown some of her best tennis in 2025, but an erratic performance in Charleston cost her, with Kalinskaya taking full advantage.

The number two seed hit 40 unforced errors and won just 48% of points on her first serve (19/40) as she struggled to find her rhythm.

Keys also converted just one of eight break point opportunities while defending just one of the five she faced in a frustrating match.