World No. 1 Serena Williams ended her stellar 2015 season early in order to fully heal from injuries. In a new interview, her coach added more insight into what is ailing the multiple Grand Slam champion.

ATP: Nadal Playing With Less Nerves, More Fun

Williams shut down her season early skipping the Asian swing and the WTA Finals in Singapore where she was defending champion.

Per the Telegraph UK, her coach Patrick Mouratoglou said that Williams is running out of cartilage in her knees.

“The cartilage is not gone, not all of it, but a big part,” Mouratoglou said. “She has bone bruises and if you keep on playing with this for too long, the next step is a stress fracture."

The issue is a similar one faced by another fellow major champion - Rafael Nadal.

“At her age, her career could really be in danger if she went too far and got more injured like Rafa 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.”

Williams missed out on her bid to complete the calendar Grand Slam in a semifinal loss to Roberta Vinci in the US Open semifinals. Mouratoglou revealed that the loss devastated Williams for some time.

"She was really, really affected, which I think is normal when you are Serena. She does everything with a hundred percent of her heart. When you do so, you are more disappointed when you don’t reach your goal.”

Currently, Williams is scheduled to compete in several year-end exhibitions and start her 2016 season playing at the Hopman Cup.

Photo Credit: Getty Images