Czech Tomas Berdych is the latest player to end his season early after announcing he would miss tournaments in Vienna and Paris because of a back injury.
The 32-year-old world number 18 still had a remote chance of qualifying for the season-ending ATP World Tour Finals for the eighth time.
“I have been playing with back pain since Wimbledon and in my last match in Beijing, I felt like it was getting worse,” Berdych said on his Twitter account.
“I was advised by my medical team to give it a few weeks rest, and to have treatment, in order to be completely healthy and pain-free and to be ready to compete at the start of 2018.”
Twelve-time grand slam champion Novak Djokovic, three-times major winner Stan Wawrinka, Kei Nishikori and Milos Raonic have all ended their seasons early to recover from injuries, while Andy Murray has not played since Wimbledon.
Four players have already qualified for the eight-man Tour Finals in London with four more spots still up for grabs.