Lindsay Davenport pulled out of her second round clash against Germany's Anna-Lena Groenefeld at the Stanford Classic on Thursday, when a lower back strain forced her to retire while trailing 5-0.
The defending champion and top seed struggled to move around the court in her first match since losing the Wimbledon final to Venus Williams earlier this month.
"I went for a backhand and it just locked on me again," said the American, who had complained of tightness in her back earlier this week.
"I knew I was in for a tough day. It's frustrating because it's so on and off. I will be really good for a few days and then just flair up the next day."
Groenefeld now plays Frenchwoman Nathalie Dechy in the quarter-finals.
The 29-year-old Davenport said she would do whatever it took to be ready for the U.S. Open, which begins of August 29.
"I still have four or five weeks before New York," she said.
"That's my main goal and if that means having to pull out of the next three weeks, I'm ready to do it. Hopefully by that time it's not an issue."
Davenport is also the top seed for next week's Acura Classic in San Diego but must be considered a doubtful starter.
"I'll make a decision by Sunday," she said. "I would say I'm below 50-50."
