Maria Sharapova's bid to become world number one was ended by a chest muscle strain on Friday as the Russian was forced to pull out of the Los Angeles Open ahead of her quarter-final match.
The top seed was set to compete against Slovakia's Daniela Hantuchova and had she won she would have become the first Russian woman to reach number one.
"I'm extremely disappointed to have withdrawn because of the re-injury of the muscle strain that I sustained late last year and which has been bothering me all week," said the 18-year-old, who first suffered the pectoral problem in October when she said she suffered from hitting too-heavy balls.
"Reaching the number one ranking has been a lifeline goal of mine and achieving it here would have be a dream come true."
Hantuchova received a walkover into the semi-finals.
"I'm sorry it happened, I was looking forward to playing her and you never want to win match like that," Hantuchova said.
Sharapova is still entered in next week's tournament in Toronto but is unlikely to take part.
She still has a chance to take the number one ranking in the next few weeks but said she was not willing to risk her health at the US Open, beginning on August 29, just to grab the top spot.
The WTA Tour has been racked by injuries over the last three week as seven of the top 11 players have either withdrawn or retired from matches in tournaments at Stanford, San Diego and Los Angeles.
Top ranked Lindsay davenport, seven-times Grand Slam champion Serena Williams and San Diego winner Mary Pierce all pulled out of Los Angeles before the tournament began.