By Mark Lamport-Stokes

April 18 (Reuters) - When Australian tennis player Sam Stosur competed in her first Olympics at the 2004 Athens Games, she readily admits she was overawed by the occasion and too shy to mix with athletes from other sports.

It was a very different story four years later, though, when one of her highlights at the Beijing Games was getting Michael Phelps to pose with her for a picture.

"I saw Michael Phelps walking across one of the pedestrian crossings and I just had to drop my bag, find my camera and get a picture with him," U.S. Open champion Stosur told Reuters with a broad grin.

"As tennis players, we don't usually do that sort of thing but I even asked Roger Federer for a picture in Beijing when he was on the practice court before me.

"I see Roger every week nearly on the circuit and you would never do that but when you're at the Olympic Games ... I don't know, something takes over and you just want to make the most of all those memories."

Stosur scaled one of her sport's pinnacles when she won her first grand slam singles title at last year's U.S. Open, crushing heavy favourite Serena Williams in the final, but the Olympics has always held a special allure for her.

The 2012 London Games will feature tennis on the fabled grass of Wimbledon and the 27-year-old Australian cannot wait to be among those vying for the medals.

"It's definitely going to be a highlight of the year no matter what happens, whether you win a medal or you go out in the first round," Stosur said.

"The two Olympics that I've been a part of already were two of the best weeks I can remember in my career ... just being part of that team and putting on your team track suit is something very special.

"Tennis is an individual sport but at the Olympics you've got your tennis team and then the bigger team of all the other athletes from your country, the medical staff and support crew all willing you on. We don't experience that at any other time."

ACCIDENTAL SINGLES

Stosur, who began her professional career better known as a doubles player with a booming serve and a powerful forehand, ended up competing in the singles competition at the 2004 Athens Games by accident.

"I was only going there to play doubles," she smiled. "I was going to be playing with Nicole Pratt and then there were some withdrawals and because I was already going to be there, I got to play in the singles.

"But I wasn't really prepared and then I found out maybe a day or two before and I lost to Chanda Rubin in three sets and then Pratty and I lost in the doubles."

Stosur, who beat three former world number ones, including Serena Williams, to reach her first grand slam singles final at the 2010 French Open, still has vivid memories of her first Olympic experience.

"You walk into your little part of the village and the level above you is all the swimmers and then across the hall you've got all the boxers running around with their gear on trying to lose weight and all that," she recalled.

"I remember just being totally overawed by the other athletes that you see. I was probably too shy, too young and too intimidated to really go out and talk to anyone at that time.

"Because our sport is so global, the tennis players were probably some of the most recognizable athletes in the village but yet for me talking to some of the other athletes, you're just in total awe and you respect them."

In Athens, Stosur rubbed shoulders with several athletes who had regular day jobs away from their chosen sport but ended up competing at the Olympics because of their burning passion and drive.

"They're doing their sport because they absolutely love it and that's their dream, to be at the Olympics," said Stosur, whose big breakthrough came in 2005 when she won the Australian Open mixed doubles title, then the U.S. Open women's doubles title later that year.

"As tennis players we get very well rewarded for what we are able to do but to see their passion and determination ... it's just incredible.

"Many of them don't get any monetary reward for what they do but they just love it and that's their sole purpose for that year or so leading in to the Games. It's very cool to be a part of that."