TOKYO (AFP) - US teenager Lydia Jacoby eclipsed teammate and Olympic champion Lilly King to claim 100m breaststroke gold on Tuesday (July 27) 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.

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

In the women's 100m backstroke, Australia's Kaylee McKeown set a new Olympic record to win the gold medal, upsetting arch-rival Regan Smith.

The 20-year-old touched in 57.47 seconds, fractionally outside her own world record, with Canada's Kylie Masse, the Rio bronze medallist, second in 57.72 seconds and American Smith third in 58.05 seconds.

"My legs were definitely hurting in the last 20," said an ecstatic McKeown.

"I'm sure it would have been pretty noticeable on the TV but I trained for that and I knew that I had a really strong back end and a really good chance to be on the podium."

Masse turned first at 50m with McKeown third, but the Australian powerfully brought it home.

McKeown had a tough lead-up to the Games, with her father dying last year from brain cancer.

But she used his memory as inspiration to swim a sensational race and smash Smith's world record at the Australian trials last month.

"It's not necessarily what I've been through," she said. "Everyone has a journey of their own and it just so happens that mine's been a really tough one.

"I wouldn't have it any other way because I don't think I'd be where I am today without all that happening."

She is also targeting the 200m backstroke gold, having recently set the fourth-quickest time in history.

Meanwhile, Britain's Tom Dean scorched to the men's 200m freestyle gold medal, overpowering teammate Duncan Scott.

Dean came from behind in the final 50m to touch in a new national record of 1:44.22, with Scott taking silver (1:44.26) ahead of Brazil's Fernando Scheffer (1:44.66).

"I knew it was going to be a dog fight. I didn't know how people were going to swim it," said an ecstatic Dean.

"I just want to say thanks so much to everyone, my mum, family, girlfriend. I'm lost for words, it's amazing."

South Korea's Hwang Sun-woo made the early running and was under world record pace at 100m but he faded as Dean and Scott made their move in the final 50m.

"Massive credit to Tom there, that was unbelievable," said Scott.

"He's a good mate out of the pool and it's great to be able to compete against him. I'm delighted with that and I'm buzzing for Deano."

Evgeny Rylov led a Russian one-two to win the men's 100m backstroke gold.

The 24-year-old hit the wall in 51.98sec to edge teammate Kliment Kolesnikov, who touched in 52 seconds, ahead of American defending champion Ryan Murphy in 52.19 seconds.