Serena Williams was the defending champion, but chose not to compete this year.
Agnieszka Radwańska was the defending champion, but lost to Nadia Petrova in the final, 6–0, 1–6, 6–3.
Sara Errani was the defending champion and she retained her title, beating Carla Suárez Navarro in the final, 6–0, 6–4.
Serena Williams was the defending champion, but lost to her sister Venus in the semifinals.
Madison Keys was the defending champion, but lost in the second round to Belinda Bencic.
Petra Kvitová was the defending champion, and successfully defended her title, defeating compatriot and fourth seed Lucie Šafářová in the final 6–7(6–8), 6–2, 6–2.
Serena Williams was the defending champion and successfully defended her title, defeating Simona Halep in the final, 6–3, 7–6(7–5).
Venus Williams was the defending champion, but lost in the first round to Daria Kasatkina.
Heather Watson was the defending champion, but lost in the quarterfinals to Johanna Larsson.
Simona Halep was the defending champion, but lost in the quarterfinals to Serena Williams.
Kiki Bertens was the defending champion and successfully defended her title, defeating Barbora Krejčíková in the final, 6–2, 6–1.
Caroline Garcia was the defending champion, but lost in the semifinals to Anastasija Sevastova in a rematch of the previous year's final.
Dominika Cibulková was the defending champion but withdrew before the tournament began.
Elena Vesnina was the defending champion, but lost in the third round to Angelique Kerber.
Johanna Konta was the defending champion, but lost in the fourth round to Venus Williams.
Simona Halep was the two-time defending champion, but lost to Karolína Plíšková in the quarterfinals.
Jeļena Ostapenko was the defending champion, but lost in the second round to Ekaterina Alexandrova.
Elina Svitolina was the two-time defending champion, but lost in the semifinals to Belinda Bencic.
Petra Kvitová was the defending champion, but lost to Kiki Bertens in the quarterfinals in a replay of the previous year’s final.
Daria Kasatkina won the women's singles title at the 2021 St. Petersburg Ladies' Trophy, after Margarita Gasparyan retired from the final with the scoreline at 6–3, 2–1.
Mona Barthel 
Sara Errani 
