Rio de Janeiro (AFP) – Katie Ledecky of the United States crushed her world record in the 400m freestyle to win Olympic gold and jump-start a potentially historic Rio campaign.

The American phenom — targeting a rare 200m, 400m and 800m Olympic freestyle treble — surged home to touch the wall in 3min 56.46sec, smashing her own world record of 3:58.37 set in the Gold Coast, Queensland in 2014.

Ledecky, who has gone from strength to strength since her surprise win in the 800m free as a 15-year-old in London, has now broken 12 world records since then.

“A goal I set after Barcelona in 2013 was 3:56 or better so to see 3:56 feels really good,” said Ledecky, who beamed when she saw the time.

She had come within a whisker of the world mark in the afternoon heats, clocking 3:58.71 — the second-fastest ever until her gold medal swim.

“It felt pretty identical to this morning with a little more pop on the back half. It just felt great.”

Jazz Carlin of Great Britain won silver — her 4:01.23 leaving her 4.77 seconds adrift. American Leah Smith took bronze in 4:01.92.