Switzerland's world number four Stan Wawrinka has withdrawn from the Rio Olympics because of a back injury.
Wawrinka, 31, aggravated the injury during the Rogers Cup in Canada, meaning five of the men's top-10 players will not participate in Rio.
Roger Federer, Tomas Berdych, Milos Raonic and Dominic Thiem have also withdrawn.
"I would like to have witnessed my third Olympic Games," Wawrinka said. "This will not be possible now."
Wawrinka suffered a shock second-round defeat at Wimbledon this year, losing to Argentine Juan Martin del Potro, who was ranked 165th in the world.