Top seed Caroline Wozniacki advanced to the semi-finals in Indian Wells on Thursday when her opponent Victoria Azarenka of Belarus retired because of a hip injury when trailing 3-0.
The eighth-seeded Azarenka, a close friend of Wozniacki, suffered the injury while trying to play a forehand during the second point of the third game in the opening set.
Denmark's Wozniacki will next meet Russian Maria Sharapova, who recovered from a dismal serving display in the second set to battle past China's Peng Shuai 6-2, 5-7, 6-3 on the showpiece stadium court.
Champion here in 2006, 16th seed Sharapova appeared to be in cruise control after sweeping through the first set and going 2-0 up in the second but she then double-faulted seven times for Peng to level the match.
The final set began with four successive breaks of serve before the Russian, regaining form with her crunching forehand, broke Peng in the seventh and ninth games to seal victory in two hours and 22 minutes.
"I turned it on for the first set and a half and then my level definitely dropped," Sharapova said courtside after serving up a total of 13 double faults.
"She took advantage of that and it was important for me to stay tough. I knew the match wasn't over until the last point."
In the day's first match, world number one Wozniacki broke Azarenka in the second game to lead 2-0 and then 30-0 on serve before her opponent summoned her trainer, laid down on a towel and had her upper left thigh massaged.
After taking a 10-minute medical timeout, Azarenka returned to play two more points for Wozniacki to take a 3-0 lead before the Belarussian forfeited the match.
"Victoria is one of my best friends on the tour, and to see her in pain on the court and to see her get injured was not nice for sure," Wozniacki said after reaching the last four at Indian Wells for a second successive year.
"I would like to have won it in another way, but I'm through to the semi-finals and I'm happy to be there."
Frenchwoman Marion Bartoli and Belgian Yanina Wickmayer advanced to the last four at Indian Wells on Wednesday.
Bartoli, at 26 the oldest player left in the draw, shrugged off a stomach bug to beat Serb Ana Ivanovic 6-4, 7-6 while Wickmayer, the 23rd seed, upset 10th-seeded Shahar Peer 6-3, 6-3.
Before the start of the Wozniacki-Azarenka match, the two players brought a Japanese flag out on to the court as a tribute to the victims of last week's devastating earthquake and tsunami in Japan.
The players had written the words: 'Our thoughts are with you! Caroline and Victoria', on the flag.
"It's just devastating to see what's going on in Japan," Wozniacki said. "We wanted to show that we're thinking about them over there. We thought this was the least we could do."
Wozniacki, who won her 13th WTA title at last month's Dubai championships, had asked her manager to find a Japanese flag, which he did after taking a two-hour drive from Indian Wells.
"I said: 'You better take your car and drive over there and get that flag to me'," the Dane added with a smile.
Venus Williams pulls out of Sony Ericsson Open
Seven-time grand slam winner Venus Williams has withdrawn from next week's Sony Ericsson Open in Miami with an abdominal injury, tournament organisers said on Thursday.
Williams suffered the injury at January's Australian Open, where she retired from her third round match, and is currently receiving rehabilitation at home.
"I am very sorry to not be playing in the Sony Ericsson Open this year. It is one of my favorite tournaments on the circuit, especially because it is so close to my home in South Florida," Williams said in a statement. "I look forward to returning to the tour soon."
Williams's sister Serena is also out of the tournament after suffering from a blood clot in one of her lungs.
The field at Miami for the tournament, which begins on Monday, features defending champion Kim Clijsters, current world number one Caroline Wozniacki and three-time grand slam champion Maria Sharapova.