Australian top seed Sam Stosur reached the quarter-finals of the Citi Open in Washington when Caroline Wozniacki was forced to stop playing in the second set due to an injured left arm on Wednesday.
Wozniacki had won the first set 7-5 but retired in the second, trailing 4-3, after having her serve broken. After the match, the Dane confirmed that in the first set at 5-5 she felt pain in her arm while hitting a backhand.
It is unclear how serious the injury is and if it will stop Wozniacki representing Denmark at the Rio Olympics next month. She is due to carry her country's flag at the opening ceremony in Rio on August 5.
Stosur's quarter-final opponent is unseeded American Jessica Pegula, who beat countryman Christina McHale 7-5, 6-2.
In some other second-round results, sixth seed Yulia Putintseva of Kazakhstan needed two hours and 22 minutes to get past Usue Arconada of the US 6-4, 4-6, 6-4 with a total of five service breaks in the final set.
Belgian seventh seed Yanina Wickmayer broke serve in the 12th game of the second set to beat Zhang Shuai of China 6-3, 7-5.
In first round action, French fourth seed Kristina Mladenovic needed three sets to down US opponent Samantha Crawford 6-2, 2-6, 6-3 while Lauren Davis of the US beat countryman Shelby Rogers 5-7, 7-5, 6-3.