Serena Williams listens to her coach Patrick Mouratoglou during a training session May 23, 2015.
MIGUEL MEDINA/AFP/Getty Images

At the age of 34, Serena Williams has accomplished so much in the world of tennis that many consider her the greatest athlete ever. But with those accomplishments come injuries and losses. And the one person who knows about that more than Williams is her longtime coach Patrick Mouratoglou.

Mouratoglou, in an interview with ESPN, gave insight into what Williams is currently dealing with and why she ended the season early after the U.S. Open, where she lost to Roberta Vinci.

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Mouratoglou spoke about a knee injury that has been plaguing Williams for some time.

"It's the same as Rafa [Rafael Nadal]," Mouratoglou said. "It's just playing for so many years, the cartilage is [almost] gone. Not all of it, but a big part, so the bones just hit themselves.

"She has bone bruises, and if you keep on playing with this for too long, too much, the next step is a stress fracture," he said.

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After having a few weeks off, Williams returned to training earlier this week, and her coach hopes that the time off did some good, not just physically but mentally. Mouratoglou said that after missing the grand slam, Williams dealt with depression.

"She was depressed," he said. "The reaction was quite strong. She was really, really affected, which I think is normal when you are Serena. She does everything with 100 percent of her heart, so you are more disappointed when you don't reach your goal. Plus, she has a level of expectation that is much higher than anyone."

As with many people who deal with depression and shut down, Williams didn't talk to her coach for two weeks.

"She doesn't want to talk when she's depressed," he said. "But it's better. Because I represent tennis, and when you are [recovering] after something like this, you have to see people who don't represent tennis. You have to change your mind. Otherwise you keep on thinking about it. I didn't force it. I know her and I understand also that she needs a break. It's fine."

As the 2016 season approaches, Mouratoglou has no doubt that Williams will be ready to step back on the court, better than ever, but says he has told her to calm down a bit and not get too excited.

"If you go too fast, then the last weeks before the tournaments, you're going to go down, and that's the moment when you need to go up, so you have to manage it," he said. "I'm happy to feel her so excited. I told her to calm down."

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Mouratoglou, who has been coaching Williams since 2012, has helped her reach eight grand slams in 14 attempts, so he may know what he's talking about.