Belinda Bencic withdrew from the Western & Southern Open because of a right forearm injury after losing the first set 6-2 Thursday against sixth-ranked Lucie Safarova.
Bencic upset top-ranked Serena Williams last weekend en route to winning the Rogers Cup in Toronto. The 12th-ranked Bencic said it's too soon to know if she will play in next week's tournament at New Haven, Connecticut.
"I don't think it's very serious," Bencic said. "I wasn't 100 percent, and to beat Lucie or even compete against her, you have to be 100 percent. I didn't want to retire. This was the first I've ever had to retire. I hope it gets better."
The forearm had bothered the 18-year-old Swiss player for a while, she said.
"Every match, it was getting tighter and tighter," Bencic said. "I was hoping that a lot of (therapy) would help, but I couldn't do it."
Safarova is scheduled to meet either 14th-seeded Elina Svitolina or unseeded Caroline Garcia in the quarterfinals.
In men's play, qualifier Alexandr Dolgopolov reached the quarterfinals with a 6-3, 3-6, 6-4 win over Jerzy Janowicz. Dolgopolov had never made it past the first round in four previous Cincinnati appearances.