Denmark's Caroline Wozniacki holds a press conference after withdrawing from the WTA Championships semi-final tennis match against Serena Williams of the US at the Khalifa International Tennis and Squash Complex in Doha on October 31, 2009. Wozniacki was forced to retire with a stomach injury at the start of the second set. AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

Denmark's Caroline Wozniacki laughs as she holds a press conference after withdrawing from the WTA Championships semi-final tennis match against Serena Williams of the US at the Khalifa International Tennis and Squash Complex in Doha on October 31, 2009. Wozniacki was forced to retire with a stomach injury at the start of the second set. AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

Denmark's Caroline Wozniacki eyes the ball before returning it to Serena Williams of the US during their WTA Championships semi-final tennis match at the Khalifa International Tennis and Squash Complex in Doha on October 31, 2009. Wozniacki was forced to retire with a stomach injury at the start of the second set. AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

Denmark's Caroline Wozniacki returns the ball to Serena Williams of the US during the WTA Championships semi-final tennis match at the Khalifa International Tennis and Squash Complex in Doha on October 31, 2009. Wozniacki was forced to retire with a stomach injury at the start of the second set. AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

Denmark's Caroline Wozniacki eyes the ball before returning it to Serena Williams of the US during their WTA Championships semi-final tennis match at the Khalifa International Tennis and Squash Complex in Doha on October 31, 2009. Wozniacki was forced to retire with a stomach injury at the start of the second set. AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

Denmark's Caroline Wozniacki eyes the ball before returning it to Serena Williams of the US during their WTA Championships semi-final tennis match at the Khalifa International Tennis and Squash Complex in Doha on October 31, 2009. Wozniacki was forced to retire with a stomach injury at the start of the second set. AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

Caroline Wozniacki of Denmark receives treatment for an injury shortly before retiring from her singles match against Serena Williams of the United States at the WTA Tennis Championships, in Doha, Qatar Saturday, Oct. 31, 2009.

Caroline Wozniacki of Denmark waves to the crowd after retiring from her singles match against Serena Williams of the United States at the WTA Tennis Championships, in Doha, Qatar Saturday, Oct. 31, 2009.

Denmark's Caroline Wozniacki squats during the WTA Championships semi-final tennis match against Serena Williams of the US at the Khalifa International Tennis and Squash Complex in Doha on October 31, 2009. Wozniacki was forced to retire with a stomach injury at the start of the second set. AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

Caroline Wozniacki of Denmark returns during her singles match against Serena Williams of the United States at the WTA Tennis Championships, in Doha, Qatar Saturday, Oct. 31, 2009.

Caroline Wozniacki of Denmark, left, hugs Serena Williams of the United States after retiring from their singles match at the WTA Tennis Championships, in Doha, Qatar Saturday, Oct. 31, 2009.

Caroline Wozniacki of Denmark serves during her singles match against Serena Williams of the United States at the WTA Tennis Championships, in Doha, Qatar Saturday, Oct. 31, 2009.

Caroline Wozniacki of Denmark returns during her singles match against Serena Williams of the United States at the WTA Tennis Championships, in Doha, Qatar Saturday, Oct. 31, 2009.

Caroline Wozniacki of Denmark receives treatment for an injury during her singles match against Serena Williams of the United States at the WTA Tennis Championships, in Doha, Qatar Saturday, Oct. 31, 2009.

Denmark's Caroline Wozniacki withdraws from the WTA Championships semi-final tennis match against Serena Williams of the US following a stomach injury at the Khalifa International Tennis and Squash Complex in Doha on October 31, 2009. AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

Denmark's Caroline Wozniacki receives medical aid during the WTA Championships semi-final tennis match against Serena Williams of the US at the Khalifa International Tennis and Squash Complex in Doha on October 31, 2009. Wozniacki was forced to retire with a stomach injury at the start of the second set. AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

A general shot of the Khalifa International Tennis and Squash Complex during the WTA Championships tennis match between Denmark's Caroline Wozniacki and Jelena Jankovic of Serbia in Doha on October 30, 2009. Jankovic won the match 6-2, 6-2.AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

Denmark's Caroline Wozniacki returns to Jelena Jankovic of Serbia during their WTA Championships tennis match at the Khalifa International Tennis and Squash Complex in Doha on October 30, 2009. Jankovic won the match 6-2, 6-2.AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

Denmark's Caroline Wozniacki returns to Jelena Jankovic of Serbia during their WTA Championships tennis match at the Khalifa International Tennis and Squash Complex in Doha on October 30, 2009. Jankovic won the match 6-2, 6-2.AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

Denmark's Caroline Wozniacki returns to Jelena Jankovic of Serbia during their WTA Championships tennis match at the Khalifa International Tennis and Squash Complex in Doha on October 30, 2009. Jankovic won the match 6-2, 6-2.AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

Denmark's Caroline Wozniacki serves to Jelena Jankovic of Serbia during their WTA Championships tennis match at the Khalifa International Tennis and Squash Complex in Doha on October 30, 2009. Jankovic won the match 6-2, 6-2.AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

Denmark's Caroline Wozniacki gestures during her tennis match against Jelena Jankovic of Serbia at the WTA Championships at the Khalifa International Tennis and Squash Complex in Doha on October 30, 2009. Jankovic won the match 6-2, 6-2.AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

Denmark's Caroline Wozniacki attends the tennis match against Jelena Jankovic of Serbia during the WTA Championships at the Khalifa International Tennis and Squash Complex in Doha on October 30, 2009. Jankovic won the match 6-2, 6-2.AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

Denmark's Caroline Wozniacki returns to Jelena Jankovic of Serbia during their WTA Championships tennis match at the Khalifa International Tennis and Squash Complex in Doha on October 30, 2009. Jankovic won the match 6-2, 6-2.AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

Denmark's Caroline Wozniacki serves to Jelena Jankovic of Serbia during their WTA Championships tennis match at the Khalifa International Tennis and Squash Complex in Doha on October 30, 2009. Jankovic won the match 6-2, 6-2.AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

Denmark's Caroline Wozniacki serves to Jelena Jankovic of Serbia during their WTA Championships tennis match at the Khalifa International Tennis and Squash Complex in Doha on October 30, 2009. Jankovic won the match 6-2, 6-2.AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

Denmark's Caroline Wozniacki serves to Jelena Jankovic of Serbia during their WTA Championships tennis match at the Khalifa International Tennis and Squash Complex in Doha on October 30, 2009. Jankovic won the match 6-2, 6-2.AFP PHOTO/KARIM JAAFAR (Photo credit should read KARIM JAAFAR/AFP/Getty Images)

Caroline Wozniacki of Denmark returns a ball to Jelena Jankovic of Serbia during their singles match at the WTA Tennis Championships, in Doha, Qatar Friday, Oct. 30, 2009.

Caroline Wozniacki of Denmark wipes her face with a towel during her singles match to Jelena Jankovic of Serbia at the WTA Tennis Championships, in Doha, Qatar Friday, Oct. 30, 2009.

Caroline Wozniacki of Denmark serves to Jelena Jankovic of Serbia during their singles match at the WTA Tennis Championships, in Doha, Qatar, Friday, Oct. 30, 2009.