World number four Stan Wawrinka has become the latest high-profile player to pull out of the Olympic tennis event.
The Swiss star, who is reportedly struggling with a back injury, joins compatriot Roger Federer in withdrawing in another blow to the tournament.
A statement issued by Wawrinka's management company said: "After a medical examination by his doctor in Geneva, the current world number four has to withdraw from the Olympic Games which will start in Rio de Janeiro later this week. Stan Wawrinka felt increasing pain during the tournament in Toronto and therefore travelled back to Switzerland in order to consult with his doctor for a complete check-up.
"Following analyses and a MRI scan, local overstress was discovered and he was advised to reduce his physical activity for an undetermined period during which he will undergo further examinations."
Switzerland had harboured hopes of medal success in singles and doubles, with Federer planning to partner Martina Hingis in the mixed doubles.
But the 17-time grand slam champion brought an early end to his season last week in order to recover fully from knee surgery in February and now Wawrinka has joined him on the sidelines.
The former Australian and French Open champion said: "I'm very sad. I was really excited about playing in Rio. After Beijing and London I was very much looking forward to playing in my third Olympic Games. I wish all the Swiss athletes the best of luck throughout the coming weeks."
Wawrinka and Federer won gold in the men's doubles in 2008 with Federer taking silver in the singles four years later in London. Half of the men's top 10 will be absent from Rio, with Tomas Berdych and Milos Raonic withdrawing because of fears over the Zika virus and Dominic Thiem also choosing to skip the event, while Rafael Nadal is a fitness doubt.