Simona Halep, a three-time runner up in Cincinnati, claimed her first win since May by overcoming Magda Linette 6-4 3-6 6-0. The 12th seed has just returned from a calf muscle tear which saw her miss the French Open and Wimbledon, and was defeated by Danielle Collins in Montreal last week.
However, at 1-4 down in the second set she required medical attention for what she described as a "sharp pain" in her right leg, but after a rain delay, the Romanian came out of the blocks to claim a third-set bagel.
Halep progressed to the Round of 32 where she will play American Jessica Pegula.
Sunday's breakthrough National Bank Open winner Camila Giorgo was beaten in straight sets by Pegula 6-2 6-2.
Former world number one Victoria Azarenka needed only one hour and nine minutes to dispose of Ludmilla Samsonova and seal her spot in the last 32.
The Belarussian triumphed 6-2 6-3 over the Russian, dominating her first serve and breaking Samsonova five times throughout the match.
Tokyo 2020 gold medalist Belinda Bencic eased past Marketa Vondrousova 6-3 7-5 continuing her bright recent run, winning in one hour and 27 minutes.
In the final match of the day's play, Caroline Garcia defeated 2017 U.S. Open champion Sloane Stephens 7-6 (9-7) 4-6 6-4 in two hours and 33 minutes.
Dayana Yastremska, Bernarda Pera, Karolina Muchova, Ons Jabeur, Elena Rybakina, Veronika Kudermetova and Coco Gauff were all winners too.