By Alan Baldwin

RIO DE JANEIRO (Reuters) - Michael Phelps, the most decorated Olympian of all time with 18 gold medals among his career haul of 22, said it was a dream come true to be chosen to carry the U.S. flag in Friday's opening ceremony at the Rio Games.

"To lead our country into this Olympics is something I honestly never thought I would have the opportunity to do," the 31-year-old told reporters on Wednesday.

"Just the honour and opportunity to do that is incredible... this has to be one of the coolest things I have ever done. To be nominated by your peers is an awesome feeling."

Phelps, who will be the first American male swimmer to compete at five Games, was chosen in a team vote to lead the delegation into the Maracana stadium in what will be his first opening ceremony.

The swimming great, who won an unprecedented eight golds at one Games in Beijing in 2008, is set to add to his haul in Rio with three individual events as well as relays.

He will not be in pool action until Sunday evening at the earliest, but in the past has shunned such ceremonies with all the standing around and staying up into the early hours.

With Rio potentially being his last Games, Phelps is determined to do things differently and savour every moment even if coach Bob Bowman had his concerns.

Phelps said he asked Bowman about how it might affect his performance on a scale of 1-10, with eight signalling clear approval. Bowman gave it a 7.8.

"I said its a no-brainer," said the swimmer, who will be only the second to carry the U.S. flag at a Summer Games opening ceremony. "I want to do it, I have to do it."

In 2000, Phelps had just wanted to make the team. Four years later, he wanted to win gold. In Beijing he aimed "to do something nobody else had done" and in London it was about making history.

"And now, I want to walk in the opening ceremony, take it all in, represent America in the best possible way and make my family proud. This time around, it's about so much more than medals," he said.

Phelps's team mates said there was nobody better to represent the country, even if men's team co-captain Nathan Adrian admitted to nerves when he put the case to the captains of other sports.

"I was stressed out. I felt if he didn't get it, it would be on my shoulders," he said.

(Editing by Ed Osmond)