Germany's top golfer Martin Kaymer cannot get to sleep at the Olympics but it is not because of noisy neighbours or medal worries. Kaymer is just so excited about being in Rio.

"When you lie in bed, it takes a while to get to sleep. You simply have so many lovely thoughts and moments swirling round in your head," said Kaymer on Monday.

The golfer was part of a group of German athletes who spontaneously sang their country's national anthem during Friday's opening ceremony at Rio's Maracana Stadium.

"At that point, I nearly had tears in my eyes," he admitted.

In the gym, for example, while training alongside burly athletes and rugby players he was left wondering "if we golfers actually do anything at all or if we are just lazy" by comparison.

And meal times shared with the rugby players left the German wide-eyed.

"Normally, you can fit a maximum of two plates on your tray in the canteen, but the rugby players load up with four or five!" he said with awe in his voice.

The rash of top golfers to withdraw from Rio has dealt a crippling blow to the credibility of the sport's return to the Olympics after a 112-year absence. The men's golf starts on Thursday.

World number one Jason Day, Dustin Johnson, Jordan Spieth and Rory McIlroy have all opted to skip the Rio Olympics, citing fears over the deadly mosquito-borne Zika virus.

But Kaymer says he cannot understand why the top names have stayed away.

"It's very, very, very difficult to understand, when someone doesn't want to be at the biggest sporting event in the world, but I think it (Zika) is a very convenient excuse," said Kaymer.

McIlroy justified his decision not to fly to Rio because of Zika, but has holidayed in Barbados, where the risk of catching the disease is just as high as Brazil.

"I am glad that I took part in this. My colleagues, I would say, have missed one of the best experiences in sport," added Kaymer.