Romania's world number five Simona Halep and Canada's world number seven Milos Raonic have pulled out of next month's Olympic Games in Rio because of concerns about the Zika virus.
Raonic, 25, was beaten in Sunday's Wimbledon final by Andy Murray.
"I am making this decision for a variety of health concerns including the uncertainty around Zika," he said.
Losing Wimbledon quarter-finalist Halep, 24, said: "I truly regret these circumstances."
More than 20 male players have withdrawn from the golf competition at Rio 2016, with many concerned about the mosquito-borne virus.
Halep and Raonic are the first tennis players to make themselves unavailable because of Zika, which is linked to brain deformities in newborn babies.
"Family is much too important for me and I can't risk not being able to have one of my own after my career in tennis is over," said Halep.
Raonic, meanwhile, wrote on Facebook: "I do not wish to impact the decision of any other athlete heading to the Games.
"I look forward to cheering on Team Canada this summer."
The United States' John Isner has pulled out to focus on his preparations for tour events, while Nick Kyrgios will not be involved because of a dispute with the Australian Olympic Committee (AOC).
BBC tennis correspondent Russell Fuller
"Raonic and Halep are the highest-ranked players to pull out of the Olympics, and the first to cite the Zika virus as a reason for their withdrawal.
"If there are other players who share their concerns, it is possible this news will make them feel more comfortable about pulling out.
"A significant number of men have already turned down the opportunity to play in Rio - Nick Kyrgios is in dispute with the Australian Olympic Committee, and others like Dominic Thiem, John Isner and Ernests Gulbis have indicated they would rather play for ranking points on the ATP Tour.
"In Gulbis' opinion, a trip to Rio is nothing more than 'tennis tourism'."