By Mark Trevelyan

RIO DE JANEIRO (Reuters) - Katinka Hosszu of Hungary won the women's Olympic 100 meters backstroke on Monday to claim her second gold of the Rio Games, edging out Kathleen Baker of the United States.

Canada's Kylie Masse and China's Fu Yuanhui tied for bronze, with the top four separated by just 0.31 seconds.

Australian world champion Emily Seebohm, swimming in an outside lane, set the early pace but Hosszu powered her way through the field in the last half of the race to touch the wall in 58.45 seconds.

Baker, who qualified fastest for the final, clocked 58.75, to finish 0.01 seconds ahead of Masse and Fu.

It was a second gold in Rio for Hosszu, the self-styled 'Iron Lady' of swimming, after her world record-breaking victory in the 400 individual medley on Saturday.

"It's really cool, I can't believe I've won the 100 back," the 27-year-old told reporters. "I know I've done the 100 back in the 400IM a million times and the goal was to make it like second nature when I came to Rio, which is how it felt.

"This time I was fresh and in great shape. I just focused on the execution and was not worrying about where I came."

Hosszu, who is also swimming in the 200 backstroke, butterfly and medley, had come away empty-handed from three previous Olympics, despite winning five world championships titles.

Seebohm, silver medalist in London in 2012, had achieved the second-fastest time in the heats but qualified for the final in seventh.

She faded in the closing stages on Monday to finish seventh.

Missy Franklin of the United States, the 2012 Olympic champion at 100, was reduced to a spectator, having failed to qualify in the event for Rio at the U.S. trials.

Franklin was in tears earlier after failing to make the final of the 200 freestyle, but has another chance in the 200 backstroke, in which she also won gold in London.

(Additional reporting by Martyn Herman; Editing by Greg Stutchbury)