Venus Williams upset Kiki Bertens as five seeds fell at the Western & Southern Open.
Former French Open and Wimbledon champion Garbine Muguruza will look to address her recent slide with a change of coach.
A finalist at Roland Garros last year, Sloane Stephens took a step towards repeating the feat by defeating Garbine Muguruza.
Naomi Osaka will retain her place at the top of the rankings after Karolina Pliskova went down to Petra Martic on Friday.
Sloane Stephens is enjoying a new lease of life with Sven Groeneveld, while Garbine Muguruza will face Elina Svitolina at Roland Garros.
Angelique Kerber did not use a recent injury setback as an excuse for her first-round exit at the hands of Anastasia Potapova in Paris.
While Simona Halep insisted fitness was not an issue ahead of the French Open, Naomi Osaka eased through at the Internazionale d'Italia.
Serena and Venus Williams will meet in the second round at the Foro Italico, 21 years after their only other clash in Rome.
Garbine Muguruza was leading 6-1 3-1 when Victoria Azarenka retired due to a leg injury in Sunday's Monterrey decider.
Not since March 2016 had Victoria Azarenka featured in a singles final but the fifth seed will end that drought thanks to Saturday's win.