As the world number 13 cruised, the 2015 runner-up and world number 18 was forced into a battle at the Charleston Open.
Caroline Wozniacki and Madison Keys enjoyed contrasting wins to move into the Charleston Open third round on Wednesday.
The 2011 champion in Charleston, Wozniacki cruised past Laura Siegemund 6-2 6-2 on the clay in South Carolina.
Wozniacki was one of 10 seeds progressing, with Mihaela Buzarnescu awaiting the Dane after the Romanian brushed past Lauren Davis 6-3 6-2.
Keys, the eighth seed and 2015 runner-up, was pushed by Tatjana Maria before getting through 7-6 (7-5) 4-6 6-4.
Jelena Ostapenko, Aryna Sabalenka, Belinda Bencic and Danielle Collins were also among the winners.
However, there were four seeded losers as Sofia Kenin, Elise Mertens, Julia Goerges and Anastasija Sevastova exited.
Monica Puig overcame Kenin 6-4 6-4 and Kaia Kanepi rallied past Mertens 0-6 6-0 7-5, while Goerges and Sevastova were beaten by Taylor Townsend and Jessica Pegula respectively.