US Open champion Dustin Johnson has joined the list of golfers to pull out of the Olympics citing the Zika virus.
His withdrawal means three of the top four players in the world will not compete in Rio in August, with Northern Ireland's Rory McIlroy and Jason Day of Australia also choosing not to play.
World number two Johnson, 32, won his first major at Oakmont in June.
"This was not an easy decision for me, but my concerns about the Zika virus cannot be ignored," he said.
"Paulina [Gretzky, Johnson's fiancee] and I plan to have more children in the near future, and I feel it would be irresponsible to put myself, her or our family at risk.
"While I am sure some will be critical of my decision, my hope is that most will understand and support it."
The Zika virus is mosquito-borne and has been linked to defects in newborn babies, prompting those looking to start a family to take extra precautions.
Olympic chiefs and the World Health Organisation have issued guidance for athletes and visitors to Rio and say the risk of catching Zika is low.
Johnson's compatriot Jordan Spieth, the world number three, has said he will go to the Games.