Borwell (right) and Kops-Jones won an ITF title in Nottingham earlier this year
British doubles number one Sarah Borwell was forced to retire from her first-ever WTA Tour semi-final in Stanford after suffering dizziness.
Borwell and American partner Raquel Kops-Jones played just one game against Lindsay Davenport and Leizel Huber.
The 30-year-old was struck in the head by a racket on Thursday during the pair's quarter-final win over Lilia Osterloh and Riza Zamelada.
"It's very upsetting, considering who we were playing against," Borwell said.
The Middlesbrough-based player, 70th in the world doubles rankings, was taken for a CAT scan following the quarter-final victory, which showed she had suffered bruising and concussion.
But despite feeling fit enough to start the match, Borwell was unable to continue, and will now miss the this week's WTA Tour event.
"It was frustrating not to be able to play." she said. "I'm now going to miss my next event in San Diego but I'm hoping to be ready for the next big one in Cincinnati."
Borwell and Kops-Jones, 27, had beaten top seeds Lisa Raymond and Rennae Stubbs on their way to the semi-final.