Katerina Siniakova of the Czech Republic recorded a stunning upset by knocking off top-seeded Serena Williams 7-6 (4), 6-2 in the second round of the Emilia-Romagna Open at Parma, Italy.

It was the sixth career win over a Top 10 player for the 25-year-old Siniakova and her first since upsetting Naomi Osaka in the third round of the 2019 French Open.

Siniakova won 15 straight points during the second set en route to dispatching the 39-year-old American in 97 minutes. She won 72.2 percent of her first-serve points.

"Not every day is really amazing," Siniakova said. "If you go to play against a player like this, you really have nothing to lose, so I came up and wanted to enjoy it."

Williams had five aces but was hurt by seven double faults. She had cruised past 17-year-old Italian qualifier Lisa Pigato 6-3, 6-2 in the first round but wasn't able to match the skill level of Siniakova, who is ranked 68th in the world and has two singles titles on the resume.

Also posting an upset was American Sloane Stephens, who recorded a 1-6, 6-4, 7-5 victory over fourth-seeded Russian Daria Kasatkina.

In other matches involving top-five seeds, No. 2 Petra Martic of Croatia rolled to a 6-4, 6-2 win over Russian Varvara Gracheva; third-seeded American Coco Gauff posted a 7-6 (6), 7-6 (7) win over Estonia's Kaia Kanepi; and fifth-seeded American Amanda Anisimova beat Slovakia's Anna Karolina Schmiedlova 6-2, 6-4.

Serbia Open

Fourth-seeded Paula Badosa of Spain cruised to a 6-0, 6-4 victory over Romania's Mihaela Buzarnescu in the second round at Belgrade, Serbia.

Fifth-seeded Nadia Podoroska of Argentina also advanced by posting a 6-2, 6-3 victory over Oceane Dodin of France.

In other top-10 matches, Russia's Anna Kalinskaya upset sixth-seeded Kristina Mladenovic of France 6-0, 7-6 (6), while No. 7 Rebecca Peterson of Sweden was a 5-7, 6-1, 6-0 winner over Serbia's Nina Stojanovic.