Germany's Mischa Zverev believes competing at a high level for so many years has taken a toll on tennis superstars Andy Murray and Novak Djokovic. Both Murray and Djokovic have failed to impress this season.

Murray hasn't played since Wimbledon as he is dealing with a hip injury. The 30-year-old was forced to skip the entire US hard-court swing. The Scot was hoping to play at the US Open but just couldn't recover in time. It remains to be seen if he will eventually decide to call it a season.

Djokovic had a stellar campaign at Wimbledon before he was forced to retire his quarter-final clash against Tomas Berdych.

The Serb, plagued by an elbow injury, later announced that he's done with the year. The 12-time Grand Slam champion is now fully committed to getting healthy and ready for the next season.

Defending US Open champion Stan Wawrinka, Canadia's Milos Raonic and Kei Nishikori, 27, were also forced to skip the US Open.

"A lot of players are injured because many of them played really good tennis for so many years," Zverev said when asked about the withdrawals ahead of the US Major, per Metro. "Let’s say almost every week they were the favorites to win the tournament, went deep into the draw. I feel like a body can break down no matter how well you take care of it, how much you train.

I think it’s human."

Anderson reaches the quarter-final

Kevin Anderson was in great spirits following his round-of-16 win over Italian Paolo Lorenzi. The South-African handed a four-set loss to Lorenzi on Sunday. .

Anderson now has been through to his second US Open quarter-final. The 31-year-old has yet to make it past the last-16 at the remaining three Grand Slams.

"Obviously it feels great right now, matching my best result here," Anderson said post-match, as revealed on US Open site. "I feel like I'm playing really good tennis. My body's healthy. It was a pretty tough loss at the beginning of this year with a few injuries."

Anderson wiser this time

Anderson is eager to learn from his 2015 US Open quarter-final defeat.

Wawrinka saw off Anderson in a three-setter to reach the semi-final two years ago.

Anderson will be battling against in-form Sam Querrey on Tuesday. Querrey has an 8-6 head-to-head record against the world No. 32.

"Definitely," Anderson said when asked whether there's anything he can take away from the 2015 quarter-final. "That match didn't go the way I wanted it. I think I'm a much better tennis player than I was back there, especially from the mental side. I feel like I've had a lot of experiences I can draw on. I'm two years older, hopefully, two years wiser.''