Serena Williams has withdrawn from the Cincinnati Masters with a recurring back injury, just days after pulling out of the Rogers Cup final.
Williams, 37, was set to face Zarina Diyas in the first round on Tuesday.
"I am so sad to withdraw as it is truly one of the tournaments I most love to play," the 23-time Grand Slam champion said.
The American has had to retire or withdraw from all five of her non-Grand Slam events this year.
During last weekend's Rogers Cup final she was left in tears because of back spasms and retired in the first set to hand the title to Canadian Bianca Andreescu.
"I came to Mason [Ohio] on Sunday and have tried everything to be ready to play tonight, and was still hopeful after my practice this morning," Williams said in a statement. "But unfortunately my back is still not right."
Her latest withdrawal comes less than two weeks before the start of the US Open, where she will be chasing a record-equalling 24th Grand Slam title.
Meanwhile, sister Venus Williams beat defending champion and fifth seed Kiki Bertens 6-3 3-6 7-6 (7-4) in a second-round battle in Ohio that lasted two hours and 17 minutes.