No. 1 Seed Serena Williams Stunned by Katerina Siniakova in Emilia-Romagna Open

Serena Williams suffered a straight-set loss to Katerina Siniakova in the second round of the Emilia-Romagna Open on Tuesday.

Williams was the top seed at the event after accepting a wild-card invitation, defeating Lisa Pigato in Round 1. She wasn't able to keep the momentum in the Round of 16 as Siniakova pulled off the 7-6(4), 6-2 upset.

"It's amazing, it means a lot to me," Siniakova told reporters after the match. ''I played [an] amazing match. It shows me that I can play like this, I can play well. I just need to try be more focused every time and to stay calm and ready for every point. I will be really happy if I could keep it like this."

Siniakova is just the 68th ranked player in the world but she was efficient against Williams, winning 72.2 percent of first serves and 64 percent of second serves. She was only broken twice in the two sets.

Williams didn't help her own cause with seven double faults and plenty of unforced errors leading to the loss.

It continues a disappointing run in the clay-court season for the American, who lost to Nadia Podoroska in her first match of the Italian Open.

The 39-year-old will prepare for Roland Garros later this month, but she hasn't won three matches in a clay-court tournament since reaching the Round of 16 at the 2018 French Open.

While still the No. 8 player in the world, Williams could have a difficult time adding to her 23 career Grand Slam titles.

Siniakova will advance to the quarterfinals in Parma, Italy to face the winner of Anna-Lena Friedsam and No. 8 seed Caroline Garcia.