By Alan Baldwin

RIO DE JANEIRO (Reuters) - Adam Peaty smashed his own world record for the second time in two days on Sunday to win 100 meters breaststroke gold and become the first British male swimmer in 28 years to win an Olympic title.

The 21-year-old world champion, who clocked 57.55 seconds in Saturday's heats, sliced another hefty chunk off his mark to finish in 57.13 and open his country's medal account at the Games.

Defending champion Cameron van der Burgh of South Africa, whose 2012 world record Peaty took in April 2015, had to settle for silver in 58.69 and Cody Miller of the United States won the bronze in 58.87.

"It's surreal to get Team GB's first gold but this is a product of seven years' hard work," said Peaty, who showed no nerves as he walked out and calmly stripped down for the biggest race of his life.

Fastest off the blocks, Peaty made the turn 0.08 of a second inside world record pace and powered down the final 50 like a man determined to exceed the expectations that have built since he won three golds at last year's world championships.

The winning margin of 1.56 seconds was as immense as his broad shoulders and the grin on his face.

"I gave it everything I've got and I did it for my country. I knew this arena would be absolutely perfect for me," said the Briton, whose parents were in the audience shedding tears of delight.

The last British male swimmer to win an Olympic gold medal was Adrian Moorhouse in the same breaststroke event at the 1988 Seoul Games.

The country's only other male champions over the last 100 years were David Wilkie in the 200m breaststroke in 1976 and Duncan Goodhew in the 100m breaststroke in 1980.

Van der Burgh said Peaty was in a league of his own.

"I am super stoked to add a silver to my gold," he said. "I knew from the warm-up I could not go 57 (seconds). So if Adam was going to plan and he did his job then...

"My body feels super strong but my stroke was not connected, my timing was off. I was a bit on, a bit off," he added.

Moorhouse, commenting for BBC television, was lost in admiration for a youngster who still lives with his parents in Uttoxeter and was taken to his first swimming lessons after screaming at the sight of bath water.

"He has the technical ability and talent to do this and can then cope with the pressure of the moment and put a bubble around himself and say, 'You know what? I should be here'," the former champion told viewers.

"He has got everything."

(Editing by Greg Stutchbury)