By Alan Baldwin

RIO DE JANEIRO (Reuters) - Ryan Murphy won the men's 100 meters backstroke gold at the Rio Olympics on Monday to extend United States swimmers' run of domination in the event to six successive Games dating back to Atlanta in 1996.

China's Xu Jiayu took the silver -- his country's first male Olympic medalist in backstroke -- and David Plummer, making his Games debut at the age of 30, secured the bronze for the United States.

The 2012 champion in the event, Matt Grevers, was unable to defend his title after failing to qualify at the U.S. trials but the team was as strong as ever with two medalists for the third successive Games.

Murphy's winning time of 51.97 seconds was the second fastest in history, just outside the world record of 51.94 set by compatriot Aaron Peirsol in 2009 when non-textile bodysuits were allowed, and an Olympic record.

"I don't even feel tired just because I'm so excited. This means everything to me. I've been swimming for 16 years and to have it come to this is just a dream come true," said the 21-year-old Murphy, a first-time Olympian.

Australia's world champion Mitch Larkin had led at the turn, 0.17 of a second inside world record pace, with Murphy only fourth.

But the Australian's challenge faded as Murphy powered through. At the touch, Plummer clocked out at 52.40 and denied Larkin a medal by just three hundredths of a second.

"I'm just overjoyed to be on the podium. To watch my teammate win, it's about as good as it gets," said Plummer, who failed to make the team in 2012 but kept going when others might have given up.

Fourth was a bitter blow for Larkin, whose world champion girlfriend Emily Seebohm finished seventh in the women's 100 backstroke, which immediately preceded the men's final.

"It was a quick race and I knew it would come down to a close finish and Ryan swum really quick and he deserves it," said Larkin. "A little bit disappointing."

(Editing by Greg Stutchbury)