Former world number one Victoria Azarenka has pulled out of the Cincinnati Open next week because of "a family matter".
The 28-year-old from Belarus only returned to tennis in June after giving birth to her son, Leo, in December.
Azarenka reached the fourth round of Wimbledon before withdrawing from last month's WTA event in Stanford, California, with a viral illness.
Defending champion Marin Cilic is also out of the Cincinnati Open.
The Croat has pulled out with the adductor injury which has sidelined him since Wimbledon, where he lost to Roger Federer in the final.
"I still don't feel at 100% to compete at the top level and to defend my title," said Cilic, who beat Andy Murray to win the Cincinnati title last year.
Murray withdrew from the Cincinnati event earlier in the week because of an ongoing hip injury.
The US Open - the final Grand Slam tournament of the year - gets under way in New York on 28 August.