Serena Williams' coach Patrick Mouratoglou has revealed the world number one's premature end to the season came about because she risked a career-ending stress fracture if she had played on with her knee injury.
Williams called time on her 2015 campaign five weeks ago, saying she needed to give her body time to recover having seen her bid for a calendar-year Grand Slam ended by a semi-final defeat to Roberta Vinci at the US Open.
''It's no secret I've played injured most of the year - whether it was my elbow, my knee, or, in the final moments after a certain match in Flushing, my heart,'' the American said in a statement.
Mouratoglou has now clarified the nature of the most significant issue, saying the 34-year-old has nearly run out of knee cartilage and could do serious damage by continuing to compete.
"The cartilage is not gone, not all of it, but a big part," the Frenchman was quoted by several newspapers as saying in Paris in his role as an ambassador for Maui Jim sunglasses.
"She has bone bruises and, if you keep on playing with this for too long, the next step is a stress fracture."
Mouratoglou, who has helped Williams secure eight grand slam titles since teaming up with her in the summer of 2012, continued: "At her age, her career could really be in danger if she went too far and got more injured like Rafa (Nadal) did in the past.
"He kept on playing with the same problem and then it got worse and he had to stop for almost a year. We don't want this to happen.
"She is 34. If she has to stop for a year, then it is really bad for her."
Source: PA
