By Alan Baldwin

RIO DE JANEIRO (Reuters) - Ryan Lochte was a top dog in the U.S. Olympic swimming team in London four years ago, but he felt more like an ageing underdog as he celebrated his 32nd birthday at the Rio Games on Wednesday.

As 31-year-old room mate Michael Phelps told a million followers on Instagram in a message accompanying a photograph of the pair of them, the friends and rivals are "the old dudes on the team now".

Lochte, who has 11 Olympic medals (five golds) from three Games compared to Phelps's record haul of 22 (18 golds) from four, repeated the picture with the additional hashtag #oldmanriver.

Sporting a striking new blue-grey hair look, with the jury out on the exact tint under the media center spotlights, Lochte said he felt the same thrill in Rio as he had at his first Games.

But he acknowledged he was in a different position with a younger generation coming through.

"The lead-up to this Games is a lot different than I guess 2012," he told reporters.

"I think I had more of a target on my back in 2012. Since the past couple of years haven't been up to par for myself and my standards, I think I'm the underdog now.

"Given what I've accomplished in the sport it doesn't seem that way, but for me personally I feel like I am the underdog. So that's really good for my mindset going into this Olympics that I'm the underdog and I have to earn my spot on the podium."

Lochte and Phelps have had some epic battles in the past, with the former losing out to Phelps in the 200 meters individual medley in 2012.

The pair have qualified for one more clash in that event in Rio, with Phelps again finishing first and Lochte -- the world champion and record holder in the event -- second in one of the highlights of the U.S. trials.

Olympic champion Lochte was also beaten into third place in the 400m IM at the trials by newcomer Chase Kalisz and Jay Litherland, saying afterwards he had pulled his groin in the earlier heats.

Lochte is no longer top of the world rankings in any discipline and will compete in just the one individual event in Rio, as well as relays. His fourth place finish in the 200m freestyle in Omaha secured a slot for the 4x200m.

(Editing by Ed Osmond)