PARMA, Italy -- Serena Williams' disappointing return to tennis continued when she lost in straight sets to 68th-ranked Katerina Siniakova in the second round of the Emilia-Romagna Open on Tuesday.

Top-seeded Williams, who accepted a wild-card invitation for the Parma tournament after losing her opening match at the Italian Open last week, was beaten 7-6 (4), 6-2.

She had beaten teenage qualifier Lisa Pigato 6-3, 6-2 in the previous round for her first victory since defeating Simona Halep in the Australian Open quarterfinals in February. After some time off, Williams then lost in straight sets in Rome to Nadia Podoroska.

Williams is preparing for the French Open in Paris, which starts on May 30.

Siniakova saved a set point at 5-4 down in the opening set and that appeared to give the Czech player confidence as after securing the tiebreak she pulled away in the second to win 16 of the last 18 points.

The 39-year-old Williams racked up double faults under pressure and Siniakova sealed the result on her third match point when the American hit a forehand long.

"It's amazing, it means a lot to me," Siniakova said. ''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."

Third-seeded Coco Gauff eliminated Kaia Kanepi 7-6 (6), 7-6 (7) in a first-round match. Second-seeded Petra Martic also got her Parma campaign off to a good start by beating Varvara Gracheva 6-4, 6-2.