Switzerland's world number four Stan Wawrinka has withdrawn from the Olympic Games because of a back injury.
Wawrinka, 31, aggravated the injury in 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Roger Federer, Tomas Berdych, Milos Raonic and Dominic Thiem have also withdrawn.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."
More to follow.