Amanda Anisimova passed her difficult opening-round test at the Western & Southern Open with flying colours, defeating world number 10 Daria Kasatkina 6-4 6-4 on Monday.
Russia's Kasatkina, who has beaten two of the world's top-seven players in the past couple of weeks, had her serve broken six times by America's Anisimova, thanks in large part to five double faults.
World number two Anett Kontaveit will face Czech qualifier Tereza Martincova in the second round after she got the better of Nuria Parrizas-Diaz 6-7 (4-7) 6-4 6-3, while world number three Maria Sakkari will play Caroline Garcia after the Frenchwoman's 6-3 6-3 win against Petra Martic.
In a pair of top-30 clashes, Veronika Kudermetova was stylish in her 6-3 6-3 victory over Barbora Krejcikova, while last year's US Open runner-up Leylah Fernandez has now lost three of her past four matches after falling 6-4 7-5 to Russia's Ekaterina Alexandrova.
Rising Swiss talent Jil Teichmann came up short in a near three-hour war of attrition against two-time Wimbledon champion Petra Kvitova 7-6 (7-2) 6-7 (6-8) 6-3, and Kazakhstan's Elena Rybakina has now won nine of her past 11 matches after defeating Mayar Sherif 6-3 6-2, with her only two losses coming against Kasatkina and Coco Gauff.
Alison Riske continued her march towards the top 20 with a 6-1 7-6 (7-2) win against Sara Sorribes Tormo, while Belarus' Victoria Azarenka knocked off 37-year-old Estonian Kaia Kanepi 6-3 4-6 6-3.
The lowest-ranked competitor in the field will face world number five Ons Jabeur in the second round after Caty McNally (world number 179) upset Aliaksandra Sasnovich 6-3 3-6 7-6 (7-2).
Australian Ajla Tomljanovic beat fellow qualifier Taylor Townsend 6-4 3-6 7-5, and in the final match of the night, Sloane Stephens hammered Alize Cornet 6-1 6-0.