Bad back sends Stan to sidelines

World No 4 Stan Wawrinka withdrew from the Olympics with a back injury on Tuesday, dealing another blow to the tennis tournament that has been hit by high-profile pullouts.

The 31-year-old is the third Swiss player to withdraw after Roger Federer and Belinda Bencic, who also scratched because of injury.

"I am very sad," said Wawrinka, the former French and Australian Open champion.