Stan Wawrinka (AP)
Rio de Janeiro - World number four Stan Wawrinka withdrew from the Olympics with a back injury on Tuesday, dealing another blow to the tennis tournament which has been hit by high-profile pull-outs.
The 31-year-old is the third Swiss player to withdraw after Roger Federer and Belinda Bencic who also scratched from the competition through injury.
"I am very sad," said Wawrinka, the former French and Australian Open champion.
"Going to Rio was close to my heart. After Beijing (2008) and London (2012), I wanted to live these Games in Brazil.
"Sadly, it's not possible. I wish good luck to all the Swiss athletes who will go to Rio and I will support them from distance."
Wawrinka aggravated his back injury in Toronto last week where he reached the semi-finals.
He won the Olympic doubles gold medal with Federer at the 2008 Games but lost in the first round of the singles in London to eventual champion Andy Murray.
World number three Federer opted out of the Games last week and shut down his 2016 season to rest a knee injury.
One day later, teenager Bencic, the world number 19, withdrew with a wrist injury.
The tennis, which starts on Saturday, has been hit by a number of pull-outs over injury, fears of the Zika virus or scheduling complaints.
Milos Raonic, Tomas Berdych, Dominic Thiem, Richard Gasquet, John Isner, Feliciano Lopez and Nick Kyrgios are not playing.
The women's singles are stripped of Simona Halep, Victoria Azarenka, who is pregnant, Bencic and Karolina Pliskova.
Maria Sharapova, silver medallist behind Serena Williams in 2012, is serving a two-year doping ban.
The men's doubles was also tainted when defending champions Bob and Mike Bryan opted out, insisting "our family's health is now our top priority".
Later Tuesday, Rafael Nadal will decide what events he will play in Rio as he makes a return from a wrist injury which has kept him off tour for two months.
The 14-time Grand Slam title winner, who practiced for the first time on Monday, is scheduled to play singles, as well as doubles with Marc Lopez and mixed with French Open champion Garbine Muguruza.
But his uncle and coach Toni said his nephew may have to rethink his schedule -- even possibly sitting out the singles.
"I don't know because that depends on who would be better for the team," Toni Nadal said when asked if his nephew will prioritise the singles.
"We are waiting until tomorrow (Tuesday) to take a decision to play all the things or not -- singles, doubles and mixed doubles."
Nadal was Olympic champion in 2008 but missed the London Games with a knee injury.
The 30-year-old is set to carry the Spanish flag at Friday's opening ceremony.
Meanwhile, a spokesman for the International Tennis Federation (ITF) told AFP that they would wait for official confirmation of Wawrinka's withdrawal from the Swiss tennis federation before a replacement is named.
Tuesday is the final day when a replacement can be drafted in from outside of the Rio Olympic site.
Swiss tennis medal hopes now may rest with the women's doubles where 35-year-old Martina Hingis will partner Timea Bacsinszky.