Serena Williams’ coach, Patrick Mouratoglou, admitted in a recent interview that the World No. 1 player is struggling with a knee injury similar to that of Rafael Nadal, a former top player in men’s tennis.

Williams ended her 2015 ATP season abruptly after the U.S. Open, where she lost to Roberta Vinci of Italy in the semifinals and missed out on becoming the second woman athlete in tennis history to win a calendar Grand Slam, where a player wins all four majors title in a single calendar year.

"It's the same as Rafa. 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,” Mouratoglou told ESPN. "She has bone bruises, and if you keep on playing with this for too long, too much, the next step is a stress fracture."

Williams’ tennis coach added that there is no other option but to rest the knee and allow it to heal on its own. Mouratoglou also stated that they do not want to put her career in danger, as there is a risk that the knee injury could get worse, similar to what Nadal has suffered the past two ATP seasons.

Nadal has dropped in the men’s singles rankings due to poor performances for the last two years. Nadal is currently ranked as World No. 6 and has not kept up with rivals Novak Djokovic, Roger Federer and Andy Murray as elite players in recent years.

Meanwhile, Williams has had an excellent 2015 season with a win-loss record of 53-3. She won the Australian Open, French Open and Wimbledon titles before losing in the final four in New York. Overall, Williams has 21 Grand Slam titles, one title short of Steffi Graf’s record of 22 majors won.

Because of her knee injury and elbow issues, Williams withdrew from the 2015 China Open last month and the WTA Finals in Singapore last week.

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