American Sloane Stephens lost 4-6, 4-6 to Elina Svitolina at the Family Circle Cup.
The 18th-ranked Stephens was upset by the 19-year-old Svitolina, who is ranked a career-best No. 35 this week.
Svitolina, who is from the Ukraine, earned her fourth career victory over a top 20 player - the third coming last week at the tournament near Miami.
Second-seeded Jelena Jankovic, the highest seeded player remaining after Serena Williams lost Tuesday, will play Wednesday night against American Lauren Davis.
Venus Williams will face Chanelle Scheepers of South Africa.