No. 9 Kerber clinched the opening set with a crosscourt forehand from the baseline on break point, then broke the American's serve in the final game which included a double-fault by Riske to set up three match points. Kerber won on her third match point when a Riske forehand went long.
Kerber beat Australian wild-card entry Jarmila Gajdosova in the first round and qualifier Alla Kudryavtseva in the second. She'll play the winner of Friday's later match between No. 28 Flavia Pennetta and Mona Barthel in the fourth round.