Serena Williams withdrew from the Cincinnati Masters on Tuesday due to back problems just days after spasms forced her to retire from the Rogers Cup final in Toronto.
The 23-time Grand Slam champion had been on fighting form in Toronto before retiring early in the first set of the final against Canadian teenager Bianca Andreescu.
Despite “trying everything” to get back on the court in Cincinnati, Williams withdrew before her second-round match.
Williams, twice champion of the Cincinnati Masters, was looking to use the tournament to fine-tune ahead of the US Open, which starts on August 26. The injury raises questions about her fitness ahead of the event.
She said: "I came to Mason on Sunday and have tried everything to be ready to play tonight, and was still hopeful after my practice this morning, but unfortunately, my back is still not right, and I know I should not take to the court.”
As the younger Williams withdrew, Venus Williams reached the third round of the Western & Southern Open with a 6-3, 3-6, 7-6 (4) upset of defending champion Kiki Bertens.
Williams, who is the oldest woman in the draw, has made her deepest Cincinnati run since reaching the 2012 semifinals.
The seven-time Grand Slam champion revealed her tactic had been to “just not go too big”.
"I just tried not to go too big, because I can go so big and I have a lot of power and it's not always easy to control it," the 39-year-old Williams said. "So I'm trying to play smart instead of going hog wild, which is extremely easy to do.
"It was just a second round, though. For me, it's just round-by-round. I have a third round to play. I'm ready for the next round."