Bob and Mike Bryan will not defend their Olympic doubles title in Rio, citing health concerns.
The twins announced their withdrawal on their Facebook page Saturday, saying that as "husbands and fathers, our family's health is now our top priority." They didn't mention the Zika virus, which has been linked to birth defects.
The Bryans won gold at the 2012 London Games. The Americans are the No. 2 doubles team and have won 16 Grand Slam title, a men's record.
Americans John Isner and Sam Querrey also are skipping the Rio Olympics, citing scheduling issues.
Wimbledon finalist Milos Raonic, Tomas Berdych and Simona Halep are among those withdrawing from Rio because of Zika.