World golf No.3 Jordan Spieth has pulled out of next month's Olympics because of fears over the Zika virus.
The American's withdrawal means the Olympic golf tournament will be without the world's top four players.
No.1 Jason Day, second-ranked Dustin Johnson and No.4 Rory McIlroy had all previously said they would not be competing in the event.
"It was out of concern at health issues we have been talking about (recently)," International Golf Federation (IGF) vice-president Ty Votaw told reporters at officials said at a news conference held at the British Open at Royal Troon, referring to the reasons for Spieth's pull-out.
Zika is a mosquito-borne virus that the World Health Organisation says is spreading rapidly in the Americas.
IGF president Peter Dawson said it was disappointing to have received so many pull-outs from the men's tournament in Rio de Janeiro.
"There is no doubt the number of withdrawals hasn't shed golf in the best light," Dawson said. "But we do understand why these individual decisions have been taken.
"Personally I think there's been something of an over-reaction to the Zika situation... but we remain confident we'll stage two very exciting and compelling golf competitions in Rio.
"We have all of the top women playing and I think the count is eight out of the top 15 men are going to be playing so we're going to have strong fields," said Dawson.
Bubba Watson, Rickie Fowler, Patrick Reed and Matt Kuchar have been announced as the four golfers in the US Olympic team.
The USA are the only men's team with four representatives. Qualifying places were determined by individual positions on the final Olympic golf rankings that concluded on Monday (NZT).
There are a total of 60 players in both the women's and men's competitions.
Golf is returning to the Games in August following an absence of 112 years.
The sport was at the 1904 Games in Missouri, USA. The women's event will make its Games debut in Rio, with world No.1 Lydia Ko set to represent New Zealand.
Frenchman Victor Dubuisson has also pulled out of the Olympics. The Ryder Cup player said he decided to withdraw because of poor form.
Open champ questions Olympic inclusion
British Open champion Zach Johnson has questioned whether the sport truly belongs on the Games schedule.
The 40-year-old American, who claimed his second major championship when he triumphed at St Andrews 12 months ago, believes minority sports are more deserving of prominence in an event that only comes around every four years.
"I don't know if golf has its place in the Olympics now," Johnson told a news conference. "We are relevant 24/7, 365 days of the year, if that's your barometer and criteria relevancy.
"I think golf fans really look forward to the majors... and the Ryder Cup in particular. I know, as a player, those are my main motivations.
"No offence to the Olympics but I'd rather be on the Ryder Cup team. As an American golfer I have that opportunity and that's what I'd rather," added Johnson.
Johnson believes that it is too early to tell how the string of high-profile withdrawals will affect golf's return to the Games.
"Will it fit in? Is there any motivation? Is there going to be any tradition?... it's yet to be seen," said the 2007 US Masters champion.
"The Olympics to me is certainly the premier event when it comes down to a lot of different sports. Those sports should be at the forefront, wrestling, all those sports that just don't get the recognition the mainstream sports get," Johnson said.
"Those athletes train essentially for three or four years for that one opportunity and one week. You can argue that basketball and soccer, do they really need to be in there either?" said Johnson.
"My guess is they want a World Cup before they want a gold medal, they'd want an NBA Championship before they want a gold medal."
Johnson, who will play alongside Australian Adam Scott and Henrik Stenson of Sweden in the first round of the Open on Thursday, says he is an avid television viewer every time the Games comes round.
"You can see the passion for the Olympics is there, pretty much regardless of any sport, but especially the ones that are just not mainstream, and I love that," he explained.