Here's a short story:

Li Na, who is a famous and wonderful tennis player, was born on February 26th , 1982 in Wuhan. She began to practice tennis at the age of 6, but read the following news: BEIJING--- China's first and only Grand Slam winner Li Na formally announced her retirement on Friday, leaving the Chinese tennis yearning for the next superstar. Following is part of her farewell and retirement letters: 2014 has become one of the most significant years in my career and my life. This year was full of amazing events, which included winning my second Grand Slam singles title at the Australian Open and sharing the extraordinary experience with my country, my team, my husband and my fans. It was also a year filled with difficult moments, such as having to deal with the inevitable ----making the decision to end my professional tennis career. The breaking news lift her fans surprised, especially as it comes on the day when the Asian Games open in Incheon, South Korea, and the Wuhan Open debuts in Li's hometown. "I know Li Na went to Shanghai and Germany in summer to have her knees treated. But I know nothing about whether she is retiring," said Ma Keqin, chief of Hubei Tennis Sport Management Center, a government-backed body in Li's hometown that pays her salary. "I felt sorry for her when she withdraw from the US Open. She must have withdrawn from the tournament because her injuries were serious. I know her well. She's been tough since childhood. She will not quit unless there is no alternative," said Xia Xiyao, Li's coach in the 1990s." Li became an icon for Chinese sports after she won the 2011 French Open, the first Asian to win a Grand Slam singles title. (End of story)

Provide your best guess for the following question. Give ONLY the guess, no other words or explanation.

For example:

Guess: <most likely guess, as short as possible; not a complete sentence, just the guess!>

Here are a few examples of questions and answers:

Question: Who announced their retirement?
Guess: Li Na

Question: When did she begin playing?
Guess: At age 6

Question: What is her job?
Guess: Tennis player

Question: What year did she retire?
Guess: 2014

Question: What year was she born?
Guess: 1982

Question: Where?
Guess: Wuhan

Question: Who coached her in the 90's
Guess: Xia Xiyao

Question: What did she win in 2011?
Guess: French Open

Question: What about in 2014?
Guess: Australian Open

Question: Where did she have her knees treated?
Guess: Shanghai and Germany

Question: When?
Guess: In the summer

Question: What agency pays her salary?
Guess: Hubei Tennis Sport Management Center

Question: Who is the Chief?
Guess: Ma Keqin

Question: What does he know about her retirement?
Guess: Nothing

Now answer the following question in the same format.

Question: Why did she quit the US Open?

Serious injuries