Bob and Mike Bryan of the?US in action against Radek Stepanek of Czech Republic and partner Nenad Zimonjic of Serbia in the mens's doubles match on day eight of the 2016 Wimbledon Championships at the All England Lawn and Croquet Club in London, United Kingdom on July 5, 2016. [Photo/IC]

Bob and Mike Bryan will not defend their Olympic tennis doubles title in Rio de Janeiro, citing health concerns.

The American twins announced their withdrawal on Facebook on 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 and been cited by several athletes in tennis and other sports who opted to skip the Summer Games that open on Friday in Brazil.

The Bryans won gold at the 2012 London Olympics, after collecting bronze in doubles at the 2008 Beijing Games. They have won 16 Grand Slam titles, a record in men's doubles.

The US Tennis Association said it was looking into its options for replacing the Bryans in Rio.

American singles players John Isner and Sam Querrey also are skipping the Olympics. Isner said he is not going because ATP ranking points are not available.

Wimbledon runner-up Milos Raonic of Canada and Wimbledon semifinalist Tomas Berdych of the Czech Republic are among several other male stars who have pulled out, citing concerns about Zika.

Associated Press