Petra Kvitova, who is playing the Bank of the West at Stanford this week, says she may have pushed herself a bit too far as she began her return to tour.
Last December, the Czech experienced a knife attack in her home, requiring extensive surgery to repair damage to her left hand.
She returned to tennis in May at the French Open and won her opening round, before entering the grass-court season and winning in Birmingham, beating Kristina Mladenovic and Lucie Safarova.
But at Wimbledon, the two-time champion fell in only the second round to Madison Brengle.