US teen Lydia Jacoby dethrones King to claim 'insane' Olympic breaststroke gold

US teenager Lydia Jacoby eclipsed teammate and Olympic champion Lilly King to claim 100m breaststroke gold Tuesday in one of the biggest upsets yet in the Tokyo pool.

The 17-year-old Games debutant shocked even herself when she stormed home from third to touch in 1min 04.95sec, staring open-mouthed at the scoreboard.

Her manic dash over the final 15m put her ahead of South Africa's Tatjana Schoenmaker (1:05.22) and a fading King on 1:05.54 -- the veteran American's first defeat since 2015 in her pet event.

King, 24, a two-time world champion and world record-holder over the distance, had been regarded as a near-certainty to defend the Olympic title she won comfortably in Rio.

Instead, Jacoby announced herself as a rising star and ended King's quest to become the first woman to win back-to-back Olympic titles.

"It was crazy. I was definitely racing for a medal. I knew that I had it in me," said the teenager, the first Olympic swimmer to hail from Alaska state.

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"I wasn't really expecting a gold medal. So, when I looked up and saw that scoreboard it was insane."

King led over the first 50m but was challenged after the turn by Schoenmaker, who set an Olympic record in the heats.

As the pair battled it out, Jacoby pulled out all the stops and unleashed a turbo-charged final 15m.

She said her focus was on staying positive after a solid but uninspiring swim in the qualifiers.

"I definitely stressed myself out yesterday so I was just trying to feel good and feel happy going into it, and I feel like I did that," she said.

King, renowned as a gritty competitor who relishes psyching out her opponents, was gracious in defeat, declaring: "The kid just had the swim of her life and I'm so proud to be her teammate."

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She said the fact her crown had been taken by a fellow American took some sting from the loss.

"I'm surprisingly OK right now. I'm very happy with my race and so excited for Lydia," she said.

"I mean, I love to see the future of American breaststroke coming up like this."

Schoenmaker said the Tokyo schedule, with finals taking place in the morning, hindered her ability to repeat the sizzling form she showed in the heats.

"We knew it was going to be a tough race and it's obviously harder doing it in the morning, waking up and being on fire, especially in 100 metres," the South African said.

"But it's the same for everyone. I'm not here to complain. I hope I give my country a bit of hope, we're going through a tough time."